I just ran into the same issue as you did, Gnome-software would tell me that debian-stable-main wasn't active and it wouldn't enable it. So I checked my sources.list to see if everything was ok, and then I went back to gnome-software, clicked on the global menu bar on the top, sources, and then found out that for some reason, gnome software didn't have the repos main, contrib, non-free enabled in that configuration window. I checked everything important in the other tabs as well, let gnome software update itself, and then I was able to install software again.
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