Hi ohkaterose, I'm sorry to hear that you haven't been able to open the game.
There is a problem for everyone with Catalina when unzipping game files, because of new security measures added.
Here is the guide to open it:
1. Control + mouse click The Fertile Crescent 0.5.4 executable and pick Show Package Contents.
2. Open Contents and control + mouse click MacOS folder and pick New Terminal at Folder (bottom of list).
3. Type "chmod +x The\ Fertile\ Crescent"
4. Close terminal and Control + mouse click executable icon again, and pick Open.
5. When asked about being an unidentified developer, click Open.
6. From now on you can double-click the executable icon whenever you want to play.
Hope that works for you!