BP_Unit has two health variables. CurrentHealth is the one you most likely want to modify. MaxHealth is primarily used to inform the health bar. These variables are both in BP_Unit’s public variables. Perhaps you cannot find them because you’re looking for them in one of BP_Unit’s child actors and have forgotten to select show inherited variables? Note that CurrentHealth is used to inform the game logic, but modifying CurrentHealth will not lead to any visible changes for the player. For this you need to queue an action to ReceiveDamage. Check out my video tutorials on the Action System if this part is unclear.