"Unable to Start" error message after downloading new pack (Sims 4)

My daughter has been using Sims 4 on Mac for several weeks with no issues. She downloaded a small add-on about a week ago, and still no issues. Today we downloaded the Vampire addition and now nothing will load. We've restarted the computer several times, repaired the app, closed and reopened, etc., and the same pop-up keeps appearing, which reads: "Unable to start: The Sims 4 is already running. [ed35e0ea:29f00e76:00000000:18a1dd51]". Any help would be appreciated.