After looking into it more, I discovered that this error has been reported in other games as well and it's usually associated with launching the game from the Itch.io app. Users reported that downloading and launching the game directly from the desktop fixes the problem for them so this might also be something you can try.
Another thread on Itch suggested that if you have "sandboxing" on, this can cause saves to not function correctly in Renpy games, and that turning "sandboxing" off, fixes the problem
~Esh