@mattygvr The fact that the issue is limited to your main account often means permissions are broken on the Documents folder, or some folder inside. In your main account, you could try renaming the Electronic Arts folder to force the game to create a new one. If that doesn't help, try the other suggestions here:
https://answers.ea.com/t5/Technical-Issues-PC/CURRENT-ISSUE-Sims-4-already-running/m-p/13276380
The master thread is for Sims 4 and a different error, but "already running" is almost always about permissions too, so everything I can think of to fix it is listed in the first post. You can of course skip the steps you've already tried, so start with the antivirus section; there's a link for changing Windows Defender's Controlled Folder Access settings.
