C:\Users\[Username]\AppData\Local\BLUS31608\ (or similar folder names depending on region/version, such as BLES02114 or Steam\user\[ID] ).
Inside, find or create a folder named: savedata
Restart Steam and launch the game to see if your progress returns.
Don’t throw your controller through the screen just yet. This error is surprisingly common across PC (Steam), PlayStation, and Xbox. The good news is that in 99% of cases, —the game just lost the path to find it.
On consoles, this error is usually tied to account syncing or external storage.