@Bluebellflora I've just spent a bit of time poking this. What I found is that my Sims folder has a "UserData.lock" file in it that, for some reason, causes this problem. To get passed it on my machine I deleted the lock file from "~/Documents/Electronic Arts/The Sims 4/".
Try using Finder to go to your Documents -> Electronic Arts -> The Sims 4 folder and rename/delete/move UserData.log, then try relaunching. If you prefer the command line then open Terminal and try:
rm -rf "~/Documents/Electronic Arts/The Sims 4/UserData.lock"
Then try to launch the game again.
