Is it possible you're clicking on/starting an old application icon, & playing a *previous version* of the game?
That may seem a silly suggestion, but it's happened to me:
Unzipping a new version, I started playing that, no problem, made a Save when I wanted to stop.
Came back to the game later, clicked on "the" icon to start the game again, went to LOAD to recover the earlier Save ... got an error message with text and a lot of machine code or something ... Repeated the same process, same result.
Then ("d'uhhh, d'ohhh" moment) I realized I'd restarted the game in a previous version - the unzipped newer version was actually in a subfolder ... I had clicked by mistake on the older game-start icon, in the folder one level up. Which could not Load a Save that was too far advanced in the game for it to be able to process (So I then banished the old stuff to a folder I labelled OLD & TIPS.)
Maybe that's a different issue from yours, as you are IN the game and trying to save, without success. ...
Anyway, I'd recommend you perhaps re-DL the game and try again. Your Saves should still be loadable, if you haven't deleted anything, I think. Good luck!