As you can see on the screenshot, Steam displays an error message and the game is greyed out in the list on the left.
If I click on the little Apple symbol on top of the list (only show games compatible with MacOS), UGCW is filtered out.
I was hoping that there would be an easy fix for that, like flagging the game as 64bit compatible on Steam. As written before, the game runs perfectly fine on Catalina.
Thanks for looking into this, I really appreciate it!