You may have reached an undocumented save limit, you may need to delete your older saves.

I had this issue - it can lead to more complex issues if you keep hanging near the limit, which is likely 1gb or about 80 saves (saves are about 8-15mb)

I've had lots of conversations with both Stadia and Larian support staff who don't or can't acknowledge the limit, but ultimately stadia needs a finite number of save slots because there's definitely storage issues.