I have found a different scenario that can cause the same error when packaging:
I was using a blueprint Decorator which used zero Blackboard Keys and I kept getting the same error.
When I added a dummy Blackboard Key Selector variable to my Decorator and made it editable I was able to package my game.
I could find no easy way to track down which Decorator was giving the error and it worked fine in Play In Editor so I resorted to creating a minimal level with one bot and a minimal Behavior Tree. I then tested each Decorator in turn until one failed to package.