Are you able to download the game through the itch.io app for Mac?
Thanks! Let me know if it works or not.
Sometimes, MacOS considers Unity Apps that are not attached to an app store (such as Apple Store, Steam or Itch.io's own app) as unauthorized. You should be able to adjust your settings so that any app can run, or download the game from the official itch.io app store.
If that doesn't work, then let me know and I'll look into it. Thank you!