Why does this script only work in studio?

This is in a LocalScript in an ImageButton in a ScreenGui in StarterGui.

player = game.Players.LocalPlayer
timer = script.Parent.Timer
enabled = true

function onClick(click)
	if player.ItemCounter["Quick Mushroom"].Value > 0 and enabled == true then
		enabled = false
		player.ItemCounter["Quick Mushroom"].Value = player.ItemCounter["Quick Mushroom"].Value - 1
		player.Character.Humanoid.WalkSpeed = 48
		timer.Visible = true
		timer.Text = "2:00"
		minutes = 2
		for seconds = 120, 0, -1 do
			local seconds2 = seconds - 60
			local secondsdisplay = seconds2
			if seconds == 119 then
				minutes = 1
			end
			if seconds == 59 then
				minutes = 0
			end
			if seconds2 < 10 then
				secondsdisplay = "0"..seconds2
			end
			if seconds < 60 then
				secondsdisplay = seconds
			end
			if seconds < 10 then
				secondsdisplay = "0"..seconds
			end
			if secondsdisplay == 60 then
				secondsdisplay = "00"
			end
			timer.Text = minutes..":"..secondsdisplay
			wait(1)
		end
		player.Character.Humanoid.WalkSpeed = 24
		timer.Visible = false
		enabled = true
	end
end

script.Parent.MouseButton1Click:connect(onClick)

The script runs just fine in studio, but when playing in-game, an error appears on Line 2 stating that "Timer" is not a valid member of ImageButton. However, when going into Studio, there is indeed a TextLabel in the ImageButton named "Timer". What's going on here?

timer = script.Parent:WaitForChild("Timer")

Try that. It might not work tho...