Is that an Origin error you see or an OS error?
Edit: Crin has blogged about this: https://sims4.crinrict.com/eng/2015/08/game-already-running/ It seems it's related to directory permissions. I wonder if Catalina's privacy changes have locked access to a folder that The Sims 4 requires.
It does look to be a macOS window that's popping up. I ran through Origin's "Repair" option to see if that would help, but it didn't. I may just need to do a clean install.
It could also have something to do with my Documents folder being managed via iCloud (though I have all the files downloaded onto my hard drive - they aren't being accessed via the cloud). It syncs across both my Macs and allows me to pick up on my saves from either device. I've never had an issue doing that in prior versions of macOS, but Catalina may have changed some elements of how that works.
