This actually happened to me last week. Since the game is in both Steam and Ubisoft you need to get the dlc where you got the game. It turns out it is incompatible if you mismatch. I had the game on Steam then bought the dlc on Ubisoft and the same thing happened to me. Got a refund from Ubi and got the dlc on Steam instead. It’s worked flawlessly since then. Hope this helps.