That error should go away if you compile the Blueprint, so that’s nothing to worry about. I think the issue here is that you either haven’t created a “Health” variable in your pawn for the HUD to read from, or you haven’t set up the Blueprint Interface the example uses to allow the HUD to read that variable in the pawn. So because it can’t find a value for health, it’s just stuck at the default zero.
Did you migrate the HUD Blueprint from the Content Examples project? If so, a Blueprint Interface asset should have come along with it. If you haven’t already, you’ll need to open up your pawn Blueprint, click the Blueprint Props button at the top, then where it says “Implemented Interfaces” in the bottom left you can use the Add button to implement “BPI_HUD_Interface_Events”. After doing that and compiling, you should see a “Get player health” function appear in the MyBlueprint panel, which you can double-click to edit. In there, you can just hook up your pawn’s health variable like this:
And in the HUD Blueprint, the relevant part for calling that Interface function and getting the player’s health is here:
So when the HUD calls that Interface Message with the pawn as the target, it’s getting the health value you hooked up and using it to update the scalar parameter responsible for filling up the bar in the health bar material. Hope that helps.