Why won't the shop open when I press x?

Quick fix I found to your problem.

local UIS = game:GetService('UserInputService')
local player = game.Players.LocalPlayer

UIS.InputBegan:connect(function(InputObject)
	local shop = script.Parent.Parent:WaitForChild('PlayerGui').ShopGui
    if InputObject.KeyCode == Enum.KeyCode.X then
        shop.Background.Visible = true
        print('X was pressed')
    end
end)

I moved the shop variable into the InputBegan function so it checks it once you have clicked a key.

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