I played without mods, and had the same issue.

I’m pretty sure it’s a bug in the native LInux version of Pathfinder: Kingmaker. The Linux build works fine, as long as you play with mouse/keyboard, but with gamepad input the kingdom management screen doesn’t work at all…

What I did in order to play it on the Deck was to tell Steam to use the Windows version via Proton instead. (Properties -> Compatibility -> Force a specific compatibility tool -> Proton (I don’t remember which version I used).