Sims 4 is already running error

@queenliaaaaaaa  Please disable McAfee, at leat temporarily, and try again to play.  As long as you don't open anything other than Sims 4, your computer should be safe.

If that doesn't help, please try playing in a new admin Windows account.  Make it a local account, as in, don't sign into Microsoft, and don't sync with any other services you normally use.  You'll be able to launch Sims 4 without reinstalling it, but your saves and other content won't be available.  That's fine; this is just a test, and if it works, that just means the cause of this error is something specific to your main account, so we know where to look next.