Why is the GUI not showing for certain devices?

When you do game:GetService("UserInputService").TouchEnabled you're trying to see if the server is using a device with a touch screen, but that's not possible. So you have to use a RemoteFunction to let the server communicate with the players, then let the server ask the player who touches the brick, and let the player respond back through a RemoteFunction.

Add a RemoteFunction into ReplicatedStorage named "GetTouchEnabled"

Then in your script put:

script.Parent.Touched:Connect(function(part)
    local plr = game.Players:GetPlayerFromCharacter(part.Parent)

    if plr then
        local TouchEnabled = game.ReplicatedStorage:WaitForChild("GetTouchEnabled"):InvokeClient(plr)

        if TouchEnabled == true then
            print(TouchEnabled)
            if plr.PlayerGui.Shop2.Enabled == false then
                plr.PlayerGui.Shop2.Enabled = true
                print("Shop Opened")
            end
        else
            print(TouchEnabled)
            if plr.PlayerGui.Shop1.Enabled == false then
                plr.PlayerGui.Shop1.Enabled = true
                print("Shop Opened")
            end
        end
    end
end)

And put a LocalScript in StarterGui with the source:

local RF = game.ReplicatedStorage:WaitForChild("GetTouchEnabled")

function RF.OnClientInvoke()
    return game:GetService("UserInputService").TouchEnabled
end