Why does tweening this make it look goofy?

II cannot get this gui to tween properly.

Error in output: Can only tween objects in the workspace (line 26)

local sign = script.Parent
local click = sign.ClickDetector


click.MouseHoverEnter:connect(function(player)
    print("Hovered")

    local signgui = Instance.new("BillboardGui",script.Parent)

    signgui.Size = UDim2.new(4,0,2,0)
    signgui.SizeOffset = Vector2.new(-0.5,3)
    signgui.AlwaysOnTop = true

    local signgui1 = Instance.new("TextLabel",signgui)

    signgui1.Size = UDim2.new(2,0,2,0)
    signgui1:TweenPosition(UDim2.new(-0.5,0,-3,0), "In", "Quad", 3)
    wait(0.5)
    signgui1:TweenPosition(UDim2.new(0.5,0,3,0), "Out", "Quad", 3)
    signgui1.Text = "Welcome, " .. player.Name .. ", to Land of Guardia by Nathen35."
    signgui1.TextScaled = true
    signgui1.TextWrapped = true

    click.MouseHoverLeave:connect(function(player)
        signgui1:TweenPosition(UDim2.new(-0.5,0,-3,0), "In", "Quad", 2)
        wait(3)
        signgui:Destroy()
    end)
end)