My ScreenGUI images don't work when I press a button. Help please?!

I have made a game and there are buttons when you press it there's a sound and there's a countdown as well. If I press the button in testing mode nothing happens only the sound plays. Here's the code:

function onClicked()
    wait(0.5)
    game.StarterGui.CountDownNo.Three.ImageTransparency = 0 -- Number Three Appears
    script.Parent.Sound:Play()
    wait(1)
    game.StarterGui.CountDownNo.Three.ImageTransparency = 1 -- Number Three Disappears
    game.StarterGui.CountDownNo.Two.ImageTransparency = 0 -- Number Two Appears
    wait(1)
    game.StarterGui.CountDownNo.Two.ImageTransparency = 1 -- Number Two Dissapears
    game.StarterGui.CountDownNo.One.ImageTransparency = 0 -- Number One Appear
    wait(1)
    game.StarterGui.CountDownNo.One.ImageTransparency = 1 -- Number One Disappears
end

script.Parent.ClickDetector.MouseClick:connect(onClicked)

Please help! I'm new to scripting.