Hello there whitmerrifield! Welcome to the forums.
Sounds like you've encountered a compatibility issue. We strongly recommend that you buy the base game and any additional content/DLC from the same launcher (Ubisoft/Steam/Epic) to prevent this from happening.
I recommend opening a support ticket so our team can take a closer look at this. You can reach our support teams here:
- Open a Support case
- Start a Live chat
- Twitter and Facebook
Please attach a full-screen image of your proof of purchase for the DLC when creating your case, as this will help our agents to investigate further.
Thanks!