My software sources seem to be messed up following the upgrade to 20.04.

I first noticed the problem when I ran pkcon update and got this output:

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fatal error: The following packages have unmet dependencies:
  libavcodec-extra: Depends: libavcodec-extra58 (= 7:4.2.4-1ubuntu0.1) but it is not going to be installed


Discover, on the other hand, doesn't seem to have trouble updating. No error messages. But I looked in Discover's Settings section and saw that there are now only sources for Snap, Flatpak, and Firmware Updates. There is nothing for the Ubuntu repos (main, universe, multiverse etc.)

I went back to the terminal and tried a couple of variations of sudo add-apt-repository ___ in the command line and kept getting messages that the components are already installed.

I'm thinking that maybe the terminal is telling me about the old bionic beaver sources being installed and Discover is simply hiding bionic and showing me focal fossa sources - which are not installed. Is that right? If so, how do I install them and how do I clear the error about unmet dependencies?

Or am I completely misunderstanding the problem here? Sigh.

(As a side note, why wouldn't the upgrade process have installed the fossa sources, anyway? I can understand why PPAs might not be there, and I even believe I remember a warning to that effect during the upgrade process; but surely the actual Ubuntu repos should be there...)

For me, the ubuntu sources etc don't appear while updates have been fetched, but they do appear.

Look at you /etc/apt/sources.list file.
It may contain both bionic and focal lines, with the bionic ones commented out (any line with a '#' at the beginning is not read)


run the command

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apt update

and list any errors you get

If none, then the output of this:

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apt policy libavcodec-extra58

My guess is that you have a PPA that you did not re-enable and update from. The PPA probably has package versions that are conflicting with the focal ones.
The upgrade process disables PPAs and third party repos beforehand. They have to be re-enabled, and updated, assuming that they have Focal packages. Standard Ubuntu upgrade fare, though obvously less easy to do if you can't see repos in Discover, and Neon does not use Kubuntu/Lubuntu/Ubuntu's sources management GUI tool ootb.

PPA repos have their info stored in /etc/apt/sources.list.d/