Nothing is in the output and I'm Trying to make a help bar. Whats wrong with it?

game.Players.LocalPlayer.Character.Humanoid.Health.Changed:connect(function()
	local health = game.Players.LocalPlayer.Character.Humanoid.Health
	local healthbar = game.StarterGui.HealthandKiGUI.BackOfHealth.Healthbar
	if health < 100 then
		healthbar.Size = UDim2.new(1, 0, 1, 0) - UDim2.new(0, 0, health/0.01, 0)
	end
end)

What is wrong with it?

You are editing the StarterGui, which is the original gui that is cloned into player's PlayerGuis, not the gui being displayed to the player. So, you actually want healthbar to be game.Players.LocalPlayer.PlayerGui.HealthandKiGUI.BackOfHealth.Healthbar