Cast is like Type Casting in programming where you are changing an object of 1 type to another like a subclass (PlayerController -> Custom PlayerController). That doesn’t seem like the case here.
Which blueprint is the event construct node coming from? Is it the CompassBP itself? You don’t need cast in that case and you would just set it. But yeah you’re probably right, it may be related to the object pin in the cast. If Event Construct is NOT in CompassBP then you need to get a reference to your CompassBP.