With my pinephone I am unable to install any software from the built-in software manager, I have tried using Manjaro and Arch ARM / Phosh. When I first got my phone this worked, now it doesn't, and I have the same problem no matter which OS I try. When I try to add an application, the download never starts even though I have WIFI and mobile connectivity.
On Mobian I've had problems with that too, so now I install everything from terminal. I've seen other people complain about it too. I don't know what's with those software managers, but it might be a good idea to report it to the developers of the app you are using.
Graphical software managers have a very long history of being crap to be honest.
In the Linux world we always use the terminal for that purpose not only because it works, but even if a graphical frontend would have worked, the terminal way is still 1k times faster and 1δΈ‡ times more convenient.