why isn't this Main Menu gui script working?

I am trying to make it were once you click play it will open the PlayFrame and close the Main it works sometimes and sometimes not like studio it works but when u play it doesn't also when i do play it in regular game it says PlayFrame isn't a valid member of Screen Gui if the PlayFrame is in the screen gui so im like :l anyways pls help

Main = script.Parent.Parent
Play = script.Parent.Parent.Parent.PlayFrame

function run()
if Main.Visible == true then
Main.Visible = false
Play.Visible = true
else
Main.Visible = true
Play.Visible = false
end

end
script.Parent.MouseButton1Down:connect(run)