Hey Unteriod-
The ReceiveHit() function is a BlueprintImplementableEvent in the editor, meaning there is no code definition for this function and it is only meant to be used in the editor. If you’re using a source build of the engine you can change this in the Actor.h file and then override the function for your Character class. An easier option would be to create a function with the parameters necessary and then inside the character blueprint create an Event Hit node and call your custom function, passing any info needed from the Hit node to your function call.
Cheers