Please go to Steam > Account Details > Manage Subscriptions - does it say your subscription was successful?
If so, next go into your Steam settings, Cloud, and check that Enable Steam Cloud is on, and in In-Game that Enable Steam Overlay is on.
Then right click on this game in your Steam Library, properties, and check Enable Steam Overlay is on there too.
If those all look okay, Please do a full **clean** re-install:
- move any valued save games elsewhere
- "uninstall" in Steam-CK2
- manually delete both the Steam/SteamApps/common/CK2 AND Documents/Paradox/CK2 folders
- re-install game in Steam, run a Steam Verify when done.
- add ck2game.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
In particular make sure there are no files or folders left under either the Steam CK2 or Documents CK2 locations, before installing again.