I think you need to delete both the save folder under the "%appdata% folder and the game folder".
And make sure you don't have any other old versions of this game in your computer, if there are you should also delete them completely
Viewing post in Can't delete saves in-game
Tried that, making sure i only had the latest version and it didn't work.
I unnistalled everything in the game's folder, in the %appdata% folder, in the itch app. I kept my old saves and persistent files elswhere so it didn't interfere and didn't put them in the game's folder after freshly installing the game again. I tried to make a new save and press "suppr" (or 'delete' in english cuz i'm french lol) and it gave me the same result ; i still can't delete saves in-game.
I don't know if this happens only to me but it's not really an issue since i can delete the saves directly in the game's and %appdata%'s RenPy folder, only a very tiny minor inconvenience.
Thank you for you time anyway and have a good day :)