Well this is very basic programming… No very basic math.
You have a variable (like “X”) and try to do something with it without giving it a number.
Basically your expression looks like this: X + 2 = Y.
Which is obviously not solvable or rather has infinite solutions but that’s not the point here.
You don’t have a default for your hud variable because that’s impossible so you have to set it before you use it.
Ideally via the event begin play node.