No longer able to start Discover from Application Launcher

[Don B. Cilly]

To uninstall Discover, try sudo apt remove --purge plasma-discover

I tried, on a 20.04 "test" system. It did not seem to remove anything else. But pay attention to the line:
The following packages will be REMOVED:
If it says it wants to remove something that looks even vaguely "critical", just say no ;·) and ask.
I get:

~$ sudo apt remove --purge plasma-discover
...
The following packages will be REMOVED:
plasma-discover* plasma-discover-backend-fwupd*
...
Do you want to continue? [Y/n] 
(Reading database ... 336542 files and directories currently installed.)
Removing plasma-discover-backend-fwupd (5.18.5-0ubuntu0.1) ...
Removing plasma-discover (5.18.5-0ubuntu0.1) ...
...
Purging configuration files for plasma-discover (5.18.5-0ubuntu0.1) ...

To install muon, sudo apt install muon should do it.

[mr_raider]

No, plasma-discover does not run without sudo.

That's a problem. Something is wrong with your permissions for the files plasma needs to run.

Here it is on my system:

[FONT=monospace][COLOR=#54FF54][B]mr_raider@HK-47[/B][/COLOR]:[COLOR=#5454FF][B]~/Downloads[/B][/COLOR]$ plasma-discover
org.kde.knewstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "
Plasma/Applet"
org.kde.knewstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "
Plasma/LookAndFeel"
org.kde.knewstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "
Plasma/LookAndFeel"
org.kde.knewstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "
Plasma/Theme"
org.kde.knewstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using.

The package type, we assume, is "
Plasma/Comic"
invalid kns backend! "/usr/share/knsrcfiles/ksysguard.knsrc" because: "Config group not found! 

Check your KNS3 installation."
adding empty sources model QStandardItemModel(0x55ebcad526b0)
QObject::connect(AbstractReviewsBackend, ResourcesModel): invalid nullptr parameter
org.kde.plasma.libdiscover: Couldn't find a category for  "fwupd-backend"
error: QSnapdRequest::ConnectionFailed "Unable to connect snapd socket: No such file or directory"
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcaf05180, name = "com.github.PintaProject.Pinta/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb2e2050, name = "com.jonathankang.Weibird/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb326b90, name = "de.create3000.titania/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb336070, name = "fr.fgrabenstaetter.DigitalAssets/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb365b00, name = "io.github.trytonvanmeer.DungeonJournal/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb36f390, name = "io.howl.Editor/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb380560, name = "mx.pwmc.Svgvi/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb2fa830, name = "net.sourceforge.electrip.Electrip/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb3bac60, name = "nl.hjdskes.gcolor3/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb3ebfb0, name = "org.freedesktop.Tuhi/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb3f4500, name = "org.gahshomar.Gahshomar/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb411770, name = "org.gnome.HexGL/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb41b430, name = "org.gnome.OfficeRunner/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb4730b0, name = "org.librehunt.Organizer/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcaf05180, name = "com.github.PintaProject.Pinta/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb2e2050, name = "com.jonathankang.Weibird/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb326b90, name = "de.create3000.titania/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb336070, name = "fr.fgrabenstaetter.DigitalAssets/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb365b00, name = "io.github.trytonvanmeer.DungeonJournal/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb36f390, name = "io.howl.Editor/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb380560, name = "mx.pwmc.Svgvi/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb2fa830, name = "net.sourceforge.electrip.Electrip/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb3bac60, name = "nl.hjdskes.gcolor3/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb3ebfb0, name = "org.freedesktop.Tuhi/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb3f4500, name = "org.gahshomar.Gahshomar/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb411770, name = "org.gnome.HexGL/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb41b430, name = "org.gnome.OfficeRunner/x86_64/stable")
could not find runtime QVector("org.gnome.Platform", "x86_64", "3.32") FlatpakResource(0x55ebcb4730b0, name = "org.librehunt.Organizer/x86_64/stable")
took really long to fetch PackageKitBackend(0x55ebcacbf360)



[/FONT][

Look through the files discover tries to open. One of the config files might be corrupt.

ALso, you can try resetting your desktop global theme back to the default . I've had programs crash on me because they can't find this or that widget or decoration.

[boxcorner]

I haven't tried to remove Discover yet ...
Meanwhile, I tried to install muon

$ sudo apt install muon
[sudo] password for ron: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
muon is already the newest version (4:5.8.0-1ubuntu5).
muon set to manually installed.
You might want to run 'apt --fix-broken install' to correct these.
The following packages have unmet dependencies:
cnijfilter-mg6200series : Depends: libpng12-0 (>= 1.2.8rel) but it is not installable
                         Depends: libtiff4 but it is not installable
E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or specify a solution).

Again, unmet dependencies reported.

ron@rdh-NUC8i3BEH:~$ sudo apt --fix-broken install
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Correcting dependencies... Done
The following packages will be REMOVED:
cnijfilter-mg6200series
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
1 not fully installed or removed.
After this operation, 8 155 kB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.
ron@rdh-NUC8i3BEH:~$

I said No because I am also trying to get printer/scanner drivers installed for my Canon PIXMMG6250.
So, I started a separate thread as I have a dependency problem there, too.
Anyway, Muon is now installed, and can be started from the Application Launcher
Many thanks for yout help!
Concerning removing Discover, I see mr_raider has replied ... something about permissions/configuration.
So perhaps I should ry to understand that first.

[boxcorner]

Discover was installed and worked fine before I opened the box, removed an 8 GB memory module and replace it with 2 x 16 GB.
Everything has worked fine since the memory upgrade, but I noticed that Discover could no longer be run from the Application Launcher.
I could not think why a memory upgrade should cause the Discover problem and agree it was probably just a coincidence, but thought I had better mention it anyway.
jlittle told me about a Discover bug which appears not to have been fixed and I have since read a similar report elsewhere.
So, when you (or someone else, sorry I don't recall who) asked me does plasma-discover run without sudo, I simply did this

ron@rdh-NUC8i3BEH:~$ plasma-discover
ron@rdh-NUC8i3BEH:~$

Now I see you ran it from ~/Downloads whereas I tried to run it from root. Did you want me to download it first?
When you wrote, "Look through the files discover tries to open. One of the config files might be corrupt."
Do you mean look through the files ... when I run it with sudo? Obviously there's nothing to look through when I try to run it without.
Also, I don't know enough yet about configuration files, for example where they are stored.
I might have known once. but if I did then it's been lost in the mists of time, which flies past way too fast at my age.
Thanks for your help.

[mr_raider]

Discover was installed and worked fine before I opened the box, removed an 8 GB memory module and replace it with 2 x 16 GB.
Everything has worked fine since the memory upgrade, but I noticed that Discover could no longer be run from the Application Launcher.
I could not think why a memory upgrade should cause the Discover problem and agree it was probably just a coincidence, but thought I had better mention it anyway.
jlittle told me about a Discover bug which appears not to have been fixed and I have since read a similar report elsewhere.
So, when you (or someone else, sorry I don't recall who) asked me does plasma-discover run without sudo, I simply did this

ron@rdh-NUC8i3BEH:~$ plasma-discover
ron@rdh-NUC8i3BEH:~$

Now I see you ran it from ~/Downloads whereas I tried to run it from root. Did you want me to download it first?
When you wrote, "Look through the files discover tries to open. One of the config files might be corrupt."
Do you mean look through the files ... when I run it with sudo? Obviously there's nothing to look through when I try to run it without.
Also, I don't know enough yet about configuration files, for example where they are stored.
I might have known once. but if I did then it's been lost in the mists of time, which flies past way too fast at my age.
Thanks for your help.

Ok. Try this, from any directory in your home, without sudo:

[FONT=monospace][COLOR=#54FF54][B]mr_raider@HP-ProBook-440-G5[/B][/COLOR]:[COLOR=#5454FF][B]~[/B][/COLOR]$ /usr/bin/plasma-discover

[/FONT]

[GreyGeek]

Ah, running plasma-discover as root, instead of running it as you from your home account, has messed up its permissions. When plasma-discover was first installed during the installation process a menu icon was put into your menu tree to give you access. When you clicked on it Discover opened up and then, sooner or later, you were asked for your account password. Now, assuming you sudo'd into root first and ran plasma-discover from the Konsole, the permissions have been messed up.

Did you try the suggestion to run
apt --fix-broken install

Also, you can, from a Konsole, use
sudo apt -f install
sudo dpkg --configure -a

repeated several times until the problem resolves itself.
Then do
sudo apt autoremove
sudo apt update
to see if the repository refreshes itself properly.
If it does then do what Don suggested.

[boxcorner]

ron@rdh-NUC8i3BEH:~$ /usr/bin/plasma-discover
ron@rdh-NUC8i3BEH:~$

[boxcorner]

Mea cupla. Seems I ran it from my Home, not root. So I believe. Sorry about that.
If I do

ron@rdh-NUC8i3BEH:~$ dir

I see all my folders/directories.

ron@rdh-NUC8i3BEH:~$ sudo apt --fix-broken install
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Correcting dependencies... Done
The following packages will be REMOVED:
cnijfilter-mg6200series
0 upgraded, 0 newly installed, 1 to remove and 4 not upgraded.
1 not fully installed or removed.
After this operation, 8 155 kB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.
ron@rdh-NUC8i3BEH:~$

I said no, because I am trying to install the drivers for my Canon PIXMA MG6250 printer/scanner.

ron@rdh-NUC8i3BEH:~$ sudo apt -f install
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Correcting dependencies... Done
The following packages will be REMOVED:
cnijfilter-mg6200series
0 upgraded, 0 newly installed, 1 to remove and 4 not upgraded.
1 not fully installed or removed.
After this operation, 8 155 kB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.
ron@rdh-NUC8i3BEH:~$

ron@rdh-NUC8i3BEH:~$ sudo apt -f install
Reading package lists... Done
Building dependency tree       
Reading state information...

Done
Correcting dependencies... Done
The following packages will be REMOVED:
cnijfilter-mg6200series
0 upgraded, 0 newly installed, 1 to remove and 4 not upgraded.
1 not fully installed or removed.
After this operation, 8 155 kB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.
ron@rdh-NUC8i3BEH:~$

ron@rdh-NUC8i3BEH:~$ sudo apt -f install
Reading package lists... Done
Building dependency tree       
Reading state information...
Done
Correcting dependencies... Done
The following packages will be REMOVED:
cnijfilter-mg6200series
0 upgraded, 0 newly installed, 1 to remove and 4 not upgraded.
1 not fully installed or removed.
After this operation, 8 155 kB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.
ron@rdh-NUC8i3BEH:~$

ron@rdh-NUC8i3BEH:~$ sudo apt autoremove
Reading package lists... Done
Building dependency tree       
Reading state information... Done
You might want to run 'apt --fix-broken install' to correct these.
The following packages have unmet dependencies:
cnijfilter-mg6200series : Depends: libpng12-0 (>= 1.2.8rel) but it is not installable
          Depends: libtiff4 but it is not installable
E: Unmet dependencies. 

Try 'apt --fix-broken install' with no packages (or specify a solution).
ron@rdh-NUC8i3BEH:~$

I have been trying to install printer/scanner drivers and kept getting that message about libpng12-0 and libtiff4.
As I understand it cnijfilter-mg6200series cannot be installed because it requires libpng12-0 and libtiff4, but I don't know how to fix it.

ron@rdh-NUC8i3BEH:~$ sudo apt update
Hit:1 [URL]http://fr.archive.ubuntu.com/ubuntu[/URL] focal InRelease
Ign:2 [URL]http://ppa.launchpad.net/michael-gruz/canon-trunk/ubuntu[/URL] focal InRelease
Err:3 [URL]http://ppa.launchpad.net/michael-gruz/canon-trunk/ubuntu[/URL] focal Release
404  Not Found [IP: 91.189.95.83 80]
Hit:4 [URL]http://fr.archive.ubuntu.com/ubuntu[/URL] focal-updates InRelease
Hit:5 [URL]http://security.ubuntu.com/ubuntu[/URL] focal-security InRelease
Get:6 [URL]http://fr.archive.ubuntu.com/ubuntu[/URL] focal-backports InRelease [98,3 kB]
Ign:7 [URL]http://fr.archive.ubuntu.com/ubuntu[/URL] focal-backports/universe amd64 DEP-11 Metadata
Get:7 [URL]http://fr.archive.ubuntu.com/ubuntu[/URL] focal-backports/universe amd64 DEP-11 Metadata [532 B]
Reading package lists...

Done             
E: The repository 'http://ppa.launchpad.net/michael-gruz/canon-trunk/ubuntu focal Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
ron@rdh-NUC8i3BEH:~$

Okay, at this point I decided to answer Y to remove cnijfilter-mg6200series
and have repeated
apt --fix-broken install
also
sudo apt -f install
sudo dpkg --configure -a
several times as you suggested.

ron@rdh-NUC8i3BEH:~$ sudo apt autoremove
Reading package lists... Done
Building dependency tree       
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 4 not upgraded.
ron@rdh-NUC8i3BEH:~$

That seems okay, but when I run apt update, I get this mess resulting from trying to install printer/scanner drivers.

ron@rdh-NUC8i3BEH:~$ sudo apt update
Ign:1 http://ppa.launchpad.net/michael-gruz/canon-trunk/ubuntu focal InRelease
Hit:2 http://fr.archive.ubuntu.com/ubuntu focal InRelease                  
Hit:3 http://fr.archive.ubuntu.com/ubuntu focal-backports InRelease                                   
Err:4 http://ppa.launchpad.net/michael-gruz/canon-trunk/ubuntu focal Release                          
404  Not Found [IP: 91.189.95.83 80]
Get:5 http://fr.archive.ubuntu.com/ubuntu focal-updates InRelease [107 kB]
Hit:6 http://security.ubuntu.com/ubuntu focal-security InRelease
Ign:7 http://fr.archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages
Ign:8 http://fr.archive.ubuntu.com/ubuntu focal-updates/main i386 Packages
Ign:9 http://fr.archive.ubuntu.com/ubuntu focal-updates/main Translation-en
Ign:10 http://fr.archive.ubuntu.com/ubuntu focal-updates/main amd64 DEP-11 Metadata
Ign:11 http://fr.archive.ubuntu.com/ubuntu focal-updates/restricted amd64 Packages
Ign:12 http://fr.archive.ubuntu.com/ubuntu focal-updates/restricted Translation-en
Ign:13 http://fr.archive.ubuntu.com/ubuntu focal-updates/universe i386 Packages
Ign:14 http://fr.archive.ubuntu.com/ubuntu focal-updates/universe amd64 Packages
Ign:15 http://fr.archive.ubuntu.com/ubuntu focal-updates/universe amd64 DEP-11 Metadata
Get:7 http://fr.archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages [221 kB]
Err:7 http://fr.archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages
File has unexpected size (213936 != 220780). Mirror sync in progress? [IP: 194.158.119.186 80]
Hashes of expected file:
- Filesize:220780 [weak]
- SHA256:00fbfe23b91c883273d0b14720aa725b5e64897db6b974ebe7b910c75dae7e4a
- SHA1:73451eafabc7bd253455e01154587478b837ca00 [weak]
- MD5Sum:f6dcf8e8d5074fcb73e7f3e81807682a [weak]
Release file created at: Wed, 01 Jul 2020 11:40:35 +0000
Get:8 http://fr.archive.ubuntu.com/ubuntu focal-updates/main i386 Packages [112 kB]
Err:8 http://fr.archive.ubuntu.com/ubuntu focal-updates/main i386 Packages

Get:9 http://fr.archive.ubuntu.com/ubuntu focal-updates/main Translation-en [86,3 kB]
Err:9 http://fr.archive.ubuntu.com/ubuntu focal-updates/main Translation-en

Get:10 http://fr.archive.ubuntu.com/ubuntu focal-updates/main amd64 DEP-11 Metadata [117 kB]
Err:10 http://fr.archive.ubuntu.com/ubuntu focal-updates/main amd64 DEP-11 Metadata

Get:11 http://fr.archive.ubuntu.com/ubuntu focal-updates/restricted amd64 Packages [25,0 kB]
Err:11 http://fr.archive.ubuntu.com/ubuntu focal-updates/restricted amd64 Packages

Get:12 http://fr.archive.ubuntu.com/ubuntu focal-updates/restricted Translation-en [6 724 B]
Err:12 http://fr.archive.ubuntu.com/ubuntu focal-updates/restricted Translation-en

Get:13 http://fr.archive.ubuntu.com/ubuntu focal-updates/universe i386 Packages [62,7 kB]
Err:13 http://fr.archive.ubuntu.com/ubuntu focal-updates/universe i386 Packages

Get:14 http://fr.archive.ubuntu.com/ubuntu focal-updates/universe amd64 Packages [116 kB]
Err:14 http://fr.archive.ubuntu.com/ubuntu focal-updates/universe amd64 Packages

Get:15 http://fr.archive.ubuntu.com/ubuntu focal-updates/universe amd64 DEP-11 Metadata [166 kB]
Err:15 http://fr.archive.ubuntu.com/ubuntu focal-updates/universe amd64 DEP-11 Metadata

Reading package lists... Done          
E: The repository 'http://ppa.launchpad.net/michael-gruz/canon-trunk/ubuntu focal Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
ron@rdh-NUC8i3BEH:~$

I will now try to remove Discover, as Don suggested.
Meanwhile, I have managed to install libpng12-0
I found this helpful https://www.linuxuprising.com/2018/0...untu-1804.html
and have also managed tp install libtiff4
I found this helpful https://tutorialforlinux.com/2020/04...buntu-20-04/3/
and I discovered this which aso I found helpful https://www.howtoforge.com/tutorial/sudo-vs-su/#logging
so I was able to run

[COLOR=Red]sudo dpkg -i cnijfilter-common_3.60-1_amd64.deb[/COLOR]

and

[COLOR=Red]sudo dpkg -i cnijfilter-mg6200series_3.60-1_amd64.deb[/COLOR]

without any dependency issues, so at last my printer works. Hooray!
Next challenge is to get the scanner driver installed.
I found this a bit confusing.

ron@rdh-NUC8i3BEH:~/my_CanonMG6250/scangearmp-mg6200series-1.80-1-deb/packages$ sudo scangearmp-common_1.80-1_amd64.deb
[sudo] password for ron: 
sudo: scangearmp-common_1.80-1_amd64.deb: command not found
ron@rdh-NUC8i3BEH:~/my_CanonMG6250/scangearmp-mg6200series-1.80-1-deb/packages$

ron@rdh-NUC8i3BEH:~/my_CanonMG6250/scangearmp-mg6200series-1.80-1-deb/packages$ sudo scangearmp-mg6200series_1.80-1_amd64.deb
sudo: scangearmp-mg6200series_1.80-1_amd64.deb: command not found
ron@rdh-NUC8i3BEH:~/my_CanonMG6250/scangearmp-mg6200series-1.80-1-deb/packages$

So, I double-clicked on scangearmp-common_1.80-1_amd64.deb and a GUI window called Package Installer popped up. I haven't seen that before! It said it required installation of 1 additional pacakge called libpango1.0-0
which I did and then installed the commom package. Why did I get commad not found when I tried to run it from the command line?
Next, I doubled-clicked on scangearmp-mg6200series_1.80-1_amd64.deb got that installed and now the scanner works as well. Hooray!
I'm noting these step as I go along, in case it might help for someone else.
Now, I need to remove Discover and try to get rid of that http://ppa.launchpad.net/michael-gru...n-trunk/ubuntu focal InRelease mess.'
Okay, Discover has been removed.
Many thanks for your help. Now I just need to find how to remove michael-gruz and hopefully that will the end of this little saga. I have learned a few more things which is always useful, so long as I can remember them next time!

[boxcorner]

When I run sudo apt remove --purge plasma-discover ...

ron@rdh-NUC8i3BEH:~$ sudo apt remove --purge plasma-discover
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
apt-config-icons-hidpi apt-config-icons-large apt-config-icons-large-hidpi libsnapd-qt1 plasma-discover-common
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
plasma-discover* plasma-discover-backend-fwupd* plasma-discover-backend-snap* plasma-discover-snap-backend*
0 upgraded, 0 newly installed, 4 to remove and 4 not upgraded.
After this operation, 2 274 kB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.
ron@rdh-NUC8i3BEH:~$

Is it safe to say YES?
Will it only remove plasma-discover and leave the packages which are no longer required?
Hmm ... I'm going to assume removing plasma-discover* plasma-discover-backend-fwupd* plasma-discover-backend-snap* plasma-discover-snap-backend* will be safe to do.

ron@rdh-NUC8i3BEH:~$ sudo apt remove --purge plasma-discover
[sudo] password for ron: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
 apt-config-icons-hidpi apt-config-icons-large apt-config-icons-large-hidpi libsnapd-qt1 plasma-discover-common
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
 plasma-discover* plasma-discover-backend-fwupd* plasma-discover-backend-snap* plasma-discover-snap-backend*
0 upgraded, 0 newly installed, 4 to remove and 8 not upgraded.
After this operation, 2 274 kB disk space will be freed.
Do you want to continue? [Y/n] y
(Reading database ... 264261 files and directories currently installed.)
Removing plasma-discover-snap-backend (5.18.5-0ubuntu0.1) ...
Removing plasma-discover-backend-snap (5.18.5-0ubuntu0.1) ...
Removing plasma-discover-backend-fwupd (5.18.5-0ubuntu0.1) ...
Removing plasma-discover (5.18.5-0ubuntu0.1) ...
Processing triggers for mime-support (3.64ubuntu1) ...
Processing triggers for dbus (1.12.16-2ubuntu2.1) ...
Processing triggers for desktop-file-utils (0.24-1ubuntu3) ...
(Reading database ... 264216 files and directories currently installed.)
Purging configuration files for plasma-discover (5.18.5-0ubuntu0.1) ...
Purging configuration files for plasma-discover-backend-snap (5.18.5-0ubuntu0.1) ...
ron@rdh-NUC8i3BEH:~$

Clicking on Discover in the Application Launcher results in a meesage saying it cannot find plasma-discover, so that seems to have worked and I just need to remove Discover from the Application Launcher.

I think I remember how to do that.
Many thanks for your help.

[claydoh]

Many thanks for your help. Now I just need to find how to remove michael-gruz and hopefully that will the end of this little saga. I have learned a few more things which is always useful, so long as I can remember them next time!

u

Undo it a number of ways. You can remove it in a similar fashion to how you added it. Just substitute the command ppa-purge for add-apt-repository and it will remove it, and revert any packages installed from it.
Which is zero in your case, as this PPA was last updated by its creator way back in 2014. It has no Focal archive.

[boxcorner]

Thanks for your help.
I found ppa-purge wasn't installed, so I did that, then

ron@rdh-NUC8i3BEH:~$ sudo ppa-purge http://ppa.launchpad.net/michael-gruz/canon-trunk/ubuntu focal InRelease
Updating packages lists
E: The repository 'http://ppa.launchpad.net/michael-gruz/canon-trunk/ubuntu focal Release' does not have a Release file.
W:  GPG error: http://cz.archive.ubuntu.com/ubuntu precise Release: The  following signatures couldn't be verified because the public key is not  available: NO_PUBKEY 40976EAF437D05B5
Warning:  apt-get update failed for some reason

ron@rdh-NUC8i3BEH:~$ sudo apt update
Hit:1  http://security.ubuntu.com/ubuntu focal-security  InRelease                                                                                                                                                   
Ign:2 http://cz.archive.ubuntu.com/ubuntu precise  InRelease                                                                                                        
Hit:3 http://cz.archive.ubuntu.com/ubuntu precise  Release                                                                                    
Hit:4 http://ppa.launchpad.net/linuxuprising/libpng12/ubuntu focal InRelease
Hit:5 http://fr.archive.ubuntu.com/ubuntu focal InRelease
Get:6 http://fr.archive.ubuntu.com/ubuntu focal-updates InRelease [107 kB]                     
Get:7 http://cz.archive.ubuntu.com/ubuntu precise Release.gpg [198 B]                          
Ign:7 http://cz.archive.ubuntu.com/ubuntu precise Release.gpg                                  
Ign:8 http://ppa.launchpad.net/michael-gruz/canon-trunk/ubuntu focal InRelease
Err:9  http://ppa.launchpad.net/michael-gruz/canon-trunk/ubuntu focal  Release                                                                                                                                       
 404  Not Found [IP: 91.189.95.83 80]
Get:6 http://fr.archive.ubuntu.com/ubuntu focal-updates InRelease [107 kB]          
Get:10 http://fr.archive.ubuntu.com/ubuntu focal-backports InRelease [98,3 kB]
Reading  package lists... Done                                                                                                                                                                                      
W: GPG error: http://cz.archive.ubuntu.com/ubuntu precise Release:  The following signatures couldn't be verified because the public key is  not available: NO_PUBKEY 40976EAF437D05B5
E: The repository 'http://ppa.launchpad.net/michael-gruz/canon-trunk/ubuntu focal Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
ron@rdh-NUC8i3BEH:~$

What do I do wrong?

[claydoh]

use ppa-purge the exact same way you used add-apt-repository to add the PPA

sudo ppa-purge http://ppa.launchpad.net/michael-gruz/canon-trunk

The extra text is not used.

If it does not work, then you can simply delete the file that corresponds to this PPA from /etc/apt/sources.list.d/ Not sure what it will be called, likely canon-trunk.list, or something involving canon or michel-gruz

sudo rm /etc/apt/sources.list.d/name-of-file.list

NOW the next question.......

What exactly are you doing here:

W: GPG error: http://cz.archive.ubuntu.com/ubuntu precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5

This repository is for a different country (this is perfectly OK) but is for Precise Pangolin 12.04.

[boxcorner]

I am currently migrating from Kubuntu 19.04 on a laptop, to Kubuntu 20.04 on a NUC mini.
I have only just recently discovered kubuntuforums.net
Previously, I used ubuntuforums.org as I used to use Ubuntu, before switching to Kubuntu.
I have had two problems since I installed Kubuntu 20.04 on my NUC mini.

(1) Discover stopped working from the Application Launcher.
(2) I had difficulty installing printer/scanner drivers for a Canon MG6250.

Somehow, during the process of trying to find how to fix the drivers problem, visiting various sites, trying various suggestions, I ended up with this mess. Initially, your reference to Precise Pangolin didn't mean much to me, as I tend to think in terms of Kubuntu 19.04, or 20.04, but having looked it up I now realise Precise Pangolin is/was an Ubuntu release, not Kubuntu. Sorry about that, but frankly I find the codenames such as Precise Pangolin superfluous. When I look at the Info Center on my NUC what it shows is: Operating System: Kubuntu 20.04

When I look up Kubuntu releases, on kubuntu.org there's a page which tells me Kubuntu 17.10 was codenamed Artful Aardvark, but no codename 20.04 is mentioned there and the downloads page on kubuntu.org for release 20.04 doesn't mention any codename either. I shall endeavour to delete the file as you kindly suggested, later. I have recently woken up here in this different timezone; a coffee might help me avoid making any more stupid mistakes! Thank you very much for your help.

[Don B. Cilly]

Kubuntu is just a "flavour" of Ubuntu. Release names are the same.
Code names are probably "superfluous" but repositories use those.
20.04 is called Focal Fossa :·/ BTW.

[boxcorner]

Thank you for the clarification and your help earlier.