The previous game CK2 is free to play on Steam now. Does that work or have a similar problem?
I don't know if that is a general bug or a glitch in your installation.
I guess check in the Bug Reports forum here if it has been reported before. If not, Please do a full **clean** re-install:
- move any valued save games elsewhere
- "uninstall" in Steam-CK3
- manually delete both the Steam/SteamApps/common/CK3 AND Documents/Paradox/CK3 folders
- re-install game in Steam, run a Steam Verify when done.
- add CK3.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
- start the game with no mods active and test with a new campaign,
In particular make sure there are no files or folders left under either the Steam CK3 or Documents CK3 locations, before installing again.
Good luck with that!