@OTPships The "cannot create a file when that file already exists" message means exactly what it says: you can't create a symlink called "The Sims 4" in a location where there's already something called "The Sims 4." That's why the first part of the symlink guide says to move the Sims 4 folder to the external drive.
If you move the folder, then start the game afterwards, you'll have created a new "The Sims 4" folder in the same location as the old one, and you'll need to delete that before creating the symlink.
If you don't see the Sims 4 folder, please make sure you're opening the correct location. Open a File Explorer window, click This PC in the left panel, double-click the C drive, then open Users > your username > Documents > Electronic Arts.