Hi,
I think the problem is that the compiler has not seen the definition of ABaseCharacter. You can tell by removing the ‘class’ (which is a habit best avoided anyway):
UPROPERTY()
TWeakObjectPtr<ABaseCharacter> PawnInstigator;
… and then you should get an error more like ‘unknown identifier: ABaseCharacter’. So you need to explicitly include the header containing ABaseCharacter at the top of this header:
#include "BaseCharacter.h"
This probably used to compile due to a different ordering of #includes.
Hope this helps,
Steve