notsav_digs Try doing a clean uninstall and reinstall of the game, as described in the accepted solution of this thread (for a different error, but the process is the same):
[CURRENT ISSUE] Sims 4 DLC shows as installed, won't load in-game | EA Forums - 11831673
And if your external drive is formatted as FAT32 or exFAT, please reformat it as NTFS before proceeding. This would erase the drive, so if you have other data you don't want to lose, move it to the internal drive first. You can see the format type by clicking Windows key-X and selecting Disk Management.
Alternately, you could partition the drive in Disk Management, formatting only the empty partition and leaving the rest as-is. Be sure to make the partition large enough for Sims 4 and updates since you won't be able to resize it later without a third-party tool. 100 GB for the game itself should be enough for a while, or 150 GB if you never want to do this again.