game already running

@veronikapoin  Try moving the entire Sims 4 folder out of Documents > Electronic Arts.  If you get an error when trying to move it, please post a screenshot, and let me know if the folder copies rather than moving.

If the folder moves but you get the same error, please create a new admin Windows account and try playing in that.  Make it local, as in, don't link it with your Microsoft account.  You'll be able to (try to) launch Sims 4 without reinstalling anything, but your saves and other content won't be available.  That can be addressed later; for now, just let me know whether the game works.  If it does, restart your computer and test again just to be sure.

Signing in under another user did end up working. Then tried restarting and going back to the original user to no avail. If I copy and move files, how do i get the game to recognize where those files are?

Moving Electronic Arts folder from the Documents folder did not solve the issue, unfortunately. There are other games in EA that are doing the same thing (Dragon Age Inquisition).

@veronikapoin  The new user means a new Documents folder, with default permissions and nothing else broken about the folder or its contents, at least at first.  If you want to use the new account, you can just move everything over to the corresponding subfolders, using exactly the same file path as your current user has, except in the new account.  For example, this:

C:\Users\original username\Documents\Electronic Arts\The Sims 4

becomes this:

C:\Users\new username\Documents\Electronic Arts\The Sims 4

To move the files between accounts, you can drop them into C:\Users\Public, which is accessible from both.

If you'd like to try to fix your main account, try running the Takeown command.  First, run a Windows search for "Command Prompt," open it, and enter this command:

reg query "HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders" /v "Personal"

The location you see is what you'll use for the command.  The command itself is this:

takeown /F "[file path to Documents folder]"

So it will probably be one of these, but double-check anyway:

takeown /F "C:\Users\username\Documents"

or

takeown /F "C:\Users\username\OneDrive\Documents"

Write your actual username for this Windows account (your main one) instead of username.  Since the syntax needs to be exactly right, please copy this command to a Notepad file, substitute in your username, and then copy that string when you're ready to use it.  Hit Windows key-X and choose either "PowerShell (Administrator)" or "Windows Terminal (Administrator)," whichever option is offered, paste the command into the window, and enter.

Then restart your computer and try again to play.

If that doesn't help, please see this post for further troubleshooting:

https://answers.ea.com/t5/Mass-Effect-Legendary-Edition/quot-A-problem-occured-while-saving-the-game-No-game-session/m-p/12689808#M17087

You'll of course apply these steps to the Documents folder your account is using, wherever Command Prompt told you that was.