Sorry, I was new to learning C ++ but i find solution your issues. Problem is OnPlayerEnterPickupBox. You forget class type.
.cpp
void APickUp::OnPlayerEnterPickUpBox(class UPrimitiveComponent* OverlappedComp, class AActor* OtherActor, class UPrimitiveComponent* OtherComp, int32 OtherBodyIndex, bool bFromSweep, const FHitResult& SweepResult)
{
Destroy();
}
.h
UFUNCTION()
void OnPlayerEnterPickUpBox(class UPrimitiveComponent* OverlappedComp, class AActor* OtherActor, class UPrimitiveComponent* OtherComp, int32 OtherBodyIndex, bool bFromSweep, const FHitResult& SweepResult);