The Memory talked about in the message is RAM not drivespace. Look to see if it is possible to upgrade your RAM. It is pretty cheap to do.
@KeyMagnolia Sims 4 will always look for its user folder in Documents > Electronic Arts , and if the folder isn't present (for example if it's been moved), the game will create a new folder in that location. The way around this is to use a symbolic link to point the game to the location where you want your user folder to be. Here's a guide:
It's best to make a backup of your user folder before trying this, just in case something goes wrong with the process. However, before you do that, I'm curious about the error you've posted. Do you see that now, with Sims 4's program files on your external drive? If so, please run a dxdiag, while the drive is plugged in, and attach it to a post.
https://help.ea.com/en/help/pc/how-to-gather-dxdiag-information/
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