[CURRENT ISSUE] Sims 4 already running

@ikcuts  So if you create a new local admin Windows account and try to play in that, you still get the "already running" error?  And I have to stress this because it's critical and some players overlook this detail, but the account absolutely needs to be local, as in, not linked to your the email address that's attached to your Microsoft account.  You'd have been asked to answer a series of security questions instead.

If you did all this and still saw the error, what antivirus do you use?  It's possible the software is blocking Sims 4, even in the new account.  It's also possible, although less likely, that you have a virus or other system issue, so please run the Malwarebytes scan described in the first post and attach the results.