I just worked through this same tutorial myself and had the same error. Here is the solution that worked for me. 1) Open BTT_FindRandomPatrol. 2) click PatrolLocation on the left-hand Variables list 3) click “Instance Editable” in the right-hand details tab. 4) compile and save
Next in the BT_Enemy (Behavior Tree Tab) 1) click the BTT_FindRandomPatrol (task in purple on the tree) 2) in the left-hand details panel change the PatrolLocation (which should now be visible since we changed to “Instance Editable”) drop-down from EnemyActor to PartrolLocation.
Save and compile everything and it should work now.