Unable to start - The Sims 4 is already running

@hinckleyanIn permissions make sure Origin.exe and TS4_x64.exe is allowed in the security.

Also have you tried removing the userData.lock file from the documents>electronic arts>sims4 folder? After that all you need to do is relaunch the game. But i have to warn you, unfortunately you loose all your families etc. so i recommend saving them to your gallery.

Another option is go to Documents > Electronic Arts > Sims 4 Folder > Rename the folder Sims 4_old > Relauch the game. It could be caused due to issue with game saves file.

Another thing to try is:

1. Go to the Documents folder (That is where your electronic arts folder will be stored)

2. Move the Electronics art folder onto your desktop, (make sure that there is no electronics folder in your documents folder- such as a copy of it) DO NOT DELETE THE ELECTRONIC ARTS FOLDER ONCE ON DESKTOP

3. Launch Sims 4...it should start...once you are in exit.

4. Go to the Documents and there you should see a new Electronic arts folder...click on it and then onto The Sims 4.

5. If you have any mods or saved games replace the Mods & Saves folders in your old electronic arts folder (the one on your desktop) with the ones in your new electronics art folder. (You can now delete the electronics art folder on your desktop)

6. Launch The Sims 4 and play like normal to see if it works

@hinckleyan  If you have Controlled Folder Access running, please either disable it or change its settings, as described here:

https://crinrict.com/blog/2019/03/windows-10-defender-controlled-folder-access.html

If that doesn't help, let me know what antivirus you use.