Well, make sure that your GameState Ref really exists.
Also, instead of passing PlayerController or Pawn by parameter, you can pass the PlayerState, considering you are inside the Character class, it stores a reference for it.
Also, abuse of prints to make sure everything is on point.
After solving the problem, if the current solution works for you, I’d like to point out that replicating logic that needs priority to every client can cause conflicts and undesirable behaviours while synching movement, variable values, and can call events/methods improperly.
Consider redesigning these blocks for the server to evaluate what is a Hit that counts and leaves the scoring to him. You can store the last hit information in a struct with lots of detailed data, for example, and gather these info to score the correct team (and lots of other game logics, too).
In a quick search I found this tutorial (UE4 Multiplayer FPS Shooter tutorial PART 4: Hit-scan line trace with health replication - YouTube) that might be useful for you to .
You can move the Health logic to a RepNotify, then call the proper team score based on a Last Hit Data.