@chantelfatoma Right, there's an extra 1 in the mount point name because the drive was, at some point, disconnected while the system was writing to it. Please eject the drive in software (right-click the drive icon), disconnect it, open a Finder window, click cmd-shift-G, and enter /Volumes in the box. You'll see an icon for your hard disk (Macintosh HD or whatever you've renamed it), plus a folder called Drive 1. Delete this folder and restart your computer, then reconnect the drive and try to install Sims 4.
In general, please make sure to always eject any external drive in software before disconnecting it. Not doing so can have consequences like what you're seeing now and can also potentially damage the drive.