I have read recently that the Mac version of Sims 4 already comes in 64 bit. So Catalina should not be the problem. There is something else that must be preventing the game from running. I myself just purchased this game for my Mac and just learned of this issue trying to play it so Im going to attempt to get a refund until this problem is resolved.
While there is no obligation to fix an issue currently only affecting a beta operating system, the official release is drawing near. Based on historical macOS releases, Catalina will likely be released 44 days from today on the 23rd of September.
The alerts regarding 32-bit application/process compatibility have been popping up since later versions of High Sierra (namely 10.13.4, which was released in March 2018). Apple has been warning developers and the public of this for nearly a year and a half at this point, so there has been ample time for Maxis to get ahead of this.
It will be important that a fix be implemented before the official release of Catalina or they're going to have lots of angry Simmers on their hands. Asking people to hold off on updating their OS won't be a reasonable or appropriate request.
I've been without The Sims 4 since June, as I've been running the Catalina Beta since day one. I knew what I was getting myself into, but if the official Catalina release rolls out and a fix still hasn't been implemented, that's on Maxis. We just need to be patient and see what happens. They're very passionate about this game and the community, so I'm confident this is high on their list of priorities.
P.S. A fix for ActivationUI was reportedly implemented in April 2019, but was rolled back because it prevented anyone running Mac OS X 10.7 Lion (released in July 2011) from playing the game. Unless Maxis comes up with a different resolution that works for everybody, they're going to have to decide which OS should take priority. A brand new one that sets precedent for future macOS versions, or an older one that was released 8 years ago and last updated 6 years ago.
