Unreal virtual function causing unresolved external error

I’m making a base gun cpp class that just has the stuff all the other guns will have. I have a virtual function called PullTrigger that doesn’t do anything, I just want to have it on the base gun so i can implement it on the derived classes without recreating it. in my header file under private, I have this code

virtual void PullTrigger();

I tried adding override after the parantheses, but it gave me an error. As is, VS is uncomplaining, but Unreal says the function has an “Unresolved external symbol”. I have no clue what that is. How do i fix this?