The Sims 4 is already running error has to do with missing rights or blocking of the user folder. The actual message is a bit misleading. In most cases the game isn’t actually already running.
Unable to start: The Sims™ 4 is already running.
The most common cause for this on Windows is the Controlled Folder Access of Windows Defender blocking the user folder. So try adding Sims 4 as an exception first. If that doesn’t help, proceed to the suggestions below.