How do I properly set text of a GUI with a function?

I am attempting to make a power level system for my ship, and I wanted a GUI to say the exact power level. I started off with something very basic, or at least I thought it was basic. The text of the GUI does not set in the function. However, if I move the setText outside of the function, the GUI updates. When the function runs, the power value does increase, but the GUI text is not changed. Any ideas?

power = script.Parent.Power
gui = game.StarterGui.ScreenGui.TextBox
function powerIncrease()
	power.Value = power.Value + 1	
	gui.Text = "Power level: "..power.Value
end
script.Parent.Add.ClickDetector.MouseClick:connect(powerIncrease)

vs this:

power = script.Parent.Power
gui = game.StarterGui.ScreenGui.TextBox
function powerIncrease()
	power.Value = power.Value + 1
end
gui.Text = "Power level: "..power.Value
script.Parent.Add.ClickDetector.MouseClick:connect(powerIncrease)

You're trying to display text on the regular Gui and not the one native to the player(s). Try this:

power = script.Parent.Power
function powerIncrease()
	for _,i in pairs(game.Players:GetPlayers()) do
		power.Value = power.Value + 1   
		i.PlayerGui.ScreenGui.TextBox.Text = "Power level: "..power.Value
	end
end
script.Parent.Add.ClickDetector.MouseClick:connect(powerIncrease)

What you chould do is, put a localscript into the StarterGui.ScreenGui.Textbox that goes like this

local textbox = script.Parent
local power = --this should lead to wherever your power object is

while wait() do
	textbox.Text = "Power lvl: "..power.Value
end

That will keep the text updated according to what the powerlevel is So now you can just worry about updating the value of the power in the other script.