When I add Android support for Unity on Linux as an additional module in Unity Hub, I find that Unity is unable to the SDK.

The problem appears to be that Unity does not set the binaries included as executable after extracting. I believe this may be fixed on newer versions of the Editor, but this is an issue for me on 2020.3.28f1.

The simple fix is to make all files in the AndroidPlayer executable. If Unity is open, close it.

Open Konsole or another terminal app.

cd ~/Unity/Hub/Editor/[Unity version]/Editor/Data/PlaybackEngines/AndroidPlayer

find . -type f -exec chmod +x {} \;

When you re-launch Unity, it should have no problem using the Android SDK.