- in PickupActor.h you don’t need to define OnPickedup_Implementation. That’s one of those things that engine does for you itself.
- in BatteryPickup.h you need define default constructor for class - I mean ABatteryPickup();. For now you do implement it in *.cpp file but don’t define in *.h file
PS: I’m myself beginner with all this UE stuff.
successfully builds for me