Kailbie How is the external drive formatted? You can check by clicking Windows key-X and selecting Disk Utility, with the drive connected of course. If it's not NTFS, please format it as such, which will erase all data on the drive.
If you have data you want to preserve on the drive and don't have space to temporarily move it to your internal drive, you can instead partition the external and format only the new partition as NTFS. I would suggest setting aside at least 120 GB for Sims 4, more if you're also going to move saves to this partition.
If the drive is already NTFS, try creating a new folder on it into which you'll install the game.
