Error: Sims 4 Already Running

@Scholastic0  Try resetting the permissions for Documents as well as the Electronic Arts and Sims 4 folders inside.  This tutorial shows your options:

https://www.tenforums.com/tutorials/88246-add-reset-permissions-context-menu-windows.html

Without knowing what the permissions settings are, I can't tell you what you'd need to change, but it's easy to compare the screenshots to what you see on your system.

If that doesn't work, just copy the Sims 4 folder to the Public folder, which you can open by clicking This PC in a File Explorer window and opening your C > Users.  In fact, do that first, just in case changing the permissions in your existing account does something unexpected.

Hi, just an update. I tried to reset my permissions but in the end doesn't seem to work at all. I'm going to transfer all of my saves to my test user isntead.

EDIT:

I've encountered the smiliar problem when I was trying to transfer files from my usual account to my test account. Before I transferred my files from my usual account, when I double-clicked on my admin test folder, it asked me run as administrator since I've already set the user as an administrator in the first place. Once I finished my transfer, I tried to run the game, but to my surprise, the same error came up! I tried to reset permissions, but the same thing happened as last time, it didn't work.

@Scholastic0  Is the same error happening in your existing account or the new one?  If it's in the new one, does it help if you don't use any files from the existing account?  You can drag the Sims 4 folder to the desktop and let the game create a new one.