Ah ok, this may be some kind of security thing that apple implemented in one of the OS updates. Some of the fixes for this get technical so we don't recommend doing them. If you still wish to attempt them, these are a couple of the ones that might work to correct this problem.
FIX A
FIX B
This one isn't exactly the same error message but this may have some info that can help.
https://forum.unity.com/threads/issue-opening-unity-mac-applications-on-mac-big-...
FIX C
Not so much of a fix but an alternative would be to use something like WINE to run the windows version of the game. https://www.winehq.org/
Again, this could get technical and we don't advise doing this unless you're confident with things like the terminal.
If all of this fails, feel free to reach out to itch support for a refund and we'll work with them to get it processed. Really sorry that this is happening and it pretty much guarantees that our future games unfortunately won't have a mac version.