My Ubuntu Software (not Ubuntu Software Center) isn't functioning any more. The only thing I get to see is the Editor's Picks.
I think it has to do with installing the GNOME Desktop Environment. This problem started when I installed GDE.
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It's the "snap" system they've gone with, although on 20.04 there are some categories at the bottom which should load. This particular post on that page may answer your question:https://forum.snapcraft.io/t/mapping-snap-store-sections-to-xdg-sections/1229/17.
In working with Snap through the gui, the previous Ubuntu Software Manager has been replaced by this system as the default. The categories may not be as plentiful as listed in the above post, and splashed across various other posts on the snapcraft forum is wording along the lines, "there is better support in 18.04."
In the event software cannot be found by any available category, there is a search bar at the top. This mean you have to search either by the name of the app you want (i.e."dolphin", "thunar", etc.) or by the name of the category (i.e "file manager").
Using the above examples, when I search for "file manager", neither Dolphin nor Thunar show up but there is text at the bottom that says 53 more matches. When I click on this, it shows those results continuing on from where you last left off. Thunar and Dolphin are both near the bottom, and none of the first six results which appear on my screen are acutally file managers (Beekeeper Studio is the first result and could arguably be considered a type of file manager, though not the type in this example).
On a side note, if you are looking for Synaptic Package Manager, it is still listed in the Universe though its last update was 19 months ago.
Ubuntu Software, a.k.a. Snap Store shows only a few software picks on the front page and nothing more when there are access errors on snapcraft.io server, which recently happens quite often. You can find the error messages in system log, or you can kill the running
snap-store process and run again
snap-store in terminal. If you see "access denied" errors in
snap-store output, then something happened again to snapcraft.io server. This is a temporary problem that goes away when they fix the server and it starts to operate normally again.