Hey OperatorCrux, I had the exact same problem.
I did however manage to get passed those error messages, but I’m stuck somewhere else now.
Maybe I can help you with this specific problem, since I struggled with exactly that for a day.
I will try lend you some visual aid maybe that will help. Basicly it’s the same advice already gave.
In the animation blueprint (mine is called ABP_Custom) check your class settings, and add the BPI animate unit interface:
After you checked that make the following changes to your custom model (mine is called BP_Unit_Custom): add a variable, just like abref, but called abref2 or something, refering to the object “ABP_Custom” (but ofcourse take your animation blueprint). Change “cast to ABP_Unit” to “Cast to ABP_your ABP name”. After doing that add a new “Animate Ability” and "Animate Movement "(uncheck context sensitive or it wont show up!).
I basicly highlighted all the key differences in the next screenshot so you can figure out what part you need to change. I hope this will help you along, goodluck 
