@Spadethebeloved I've merged your post with the master thread for this error. Please see the first post on page one.
Since you've instaleld Sims 4 through Steam, you'd also need to launch or repair the game through Steam. The repair option is called "Verify integrity of the game files" and should be somewhere in Properties, perhaps in Local Files. It appparently moves around some.
@Spadethebeloved There are a number of suggestions in the first post, and I wasn't aware you'd tried all of them. Please make sure you have, as they're the most common fixes for this error.
If none of those interventions work(ed), you can try the ones here:
https://answers.ea.com/t5/Technical-Issues-PC/CURRENT-ISSUE-Sims-4-already-running/m-p/13276380
