My script is only changing gui size once when i press Q how can i fix this?

GENERAL PRACTICE

You've already defined the key for the Enum, though you didn't use it in the UIS check

Make "stamina" a Local Variable

You have an unnecessary if-then statement

if stamina == 100 then
end

Your stamina can drop below 0, so you should have a check to make sure that this cannot occur, or else at some point, the gui will not be visible to the player for a long period of time if they spam the "Q" Key.

ISSUES

Your set amounts for the Size are the exact same for losing stamina and gaining it back. Instead, use a percentage to calculate the frame change.

REVISED LOCAL SCRIPT

local play = game:GetService("Players").LocalPlayer
local user = game:GetService("UserInputService")
local key = Enum.KeyCode.Q
local Orange = script.Parent 
local stamina = 100

user.InputBegan:Connect(function(input, event)
	if input.UserInputType == Enum.UserInputType.Keyboard and input.KeyCode == key then
		if stamina >= 10 then
			stamina = stamina - 10  
			Orange.Size = UDim2.new((stamina / 100), 0, 1, 0)
			if stamina < 100 then
				wait(2) 
				stamina = stamina + 10 
				Orange.Size = UDim2.new((stamina / 100), 0, 1, 0)
			end
		end
	end
end)