You need to cast the score variable from your MyCharacter BP to your HUD. That way, your MyCharacter BP does all the math, and your HUD just displays it. There isn’t any need to cast from the HUD. I did a quick mockup below.
The Level Blueprint adds 1 to the variable ScoreVar each time I press the F key. It then Casts that result to the HUD. The HUD Blueprint then draws the score to the bottom right of the screen. Everytime I hit the F key, it updates. Does this help?