@lisamwitt if you do find the backup somewhere, this post might be worth trying out: https://answers.ea.com/t5/Technical-Issues-Mac/Info-The-Sims-4-macOS-Catalina-Troubleshooting/m-p/8291917#M20883
I just spent the afternoon going through this step by step (had to do the deleting of the UserData.lock file more than once, fyi). So I absolutely understand the frustration and I’m sorry the system’s being such a nuisance. I hope that troubleshooting post helps (there are some more solutions in the thread too, in case it doesn’t). Catalina apparently doesn’t play well with anything that’s not Mac native. (Insert my shocked face. I love my Mac but honestly, ugh)