Can someone help? [ANSWERED]

I'm having trouble with a script. Whenever I click my GUI, it doesn't turn visible.

I used this script:

script.Parent.MouseButton1Click:connect(function()
	MainShop = script.Parent.Parent.MainShop
	if mainShop.Visible ~= false then
		MainShop.Visible = true
		script.Parent.Text = "Close"
	else
		MainShop.Visible = false
		script.Parent.Text = "Open"
	end
end)

In output it says: Players.Player1.PlayerGui.Shop.Button.Script:3: attempt to index global 'mainShop' (a nil value)

Any help?