[2021] CKIII won't start [Linux]

That error seems like a bad installation. If you run a Steam Verify operation against the game does it find anything to fix?

Please attach your Documents/Paradox/CK3/pdx_settings.txt .
From your Documents/Paradox/CK3/logs/ folder, attach system.log , error.log

Also in Steam please click on Steam-Help-System Info, and paste everything there into a text file and attach that here.  

The settings and log files should be under /home/user/.paradoxinteractive/Crusader Kings III/  

Given that the game loads up to some degree there must be the settings file and some logs. Maybe Home is different on your machine, can you find pdx_settings.txt anywhere?