如何展示部件的长度

我在测试 billboard GUI 时遇到了问题，它能够展示部件的长度，但是它并没有展示任何的长度，一直处于零的状态。我尝试使用循环来测试，但是没有结果，然后尝试使用 RunService，但也没有结果，所以我真的很困惑，因为输出中没有任何错误，请帮帮我

代码：

    function setBill(text, parent)
    local Loop = {}

    local BillboardGui = Instance.new("BillboardGui")
    local ScreenGui = Instance.new("ScreenGui")
    local Frame = Instance.new("Frame")
    local TextLabel = Instance.new("TextLabel")

    BillboardGui.Parent = parent
    BillboardGui.ZIndexBehavior = Enum.ZIndexBehavior.Sibling
    BillboardGui.Active = true
    BillboardGui.AlwaysOnTop = true
    BillboardGui.LightInfluence = 1.000
    BillboardGui.Size = UDim2.new(0, 205, 0, 55)
    BillboardGui.ResetOnSpawn = false

    ScreenGui.Parent = BillboardGui
    ScreenGui.ZIndexBehavior = Enum.ZIndexBehavior.Sibling

    Frame.Parent = BillboardGui
    Frame.BackgroundColor3 = Color3.fromRGB(71, 71, 71)
    Frame.BackgroundTransparency = 1
    Frame.BorderColor3 = Color3.fromRGB(255, 255, 255)
    Frame.BorderSizePixel = 5
    Frame.Size = UDim2.new(0, 150, 0, 50)

    TextLabel.Parent = Frame
    TextLabel.BackgroundColor3 = Color3.fromRGB(255, 255, 255)
    TextLabel.BackgroundTransparency = 1.000
    TextLabel.Size = UDim2.new(0, 150, 0, 50)
    TextLabel.Font = Enum.Font.Gotham
    TextLabel.TextColor3 = Color3.fromRGB(255, 255, 255)
    TextLabel.TextScaled = false
    TextLabel.TextSize = 24.000
    TextLabel.TextWrapped = true

    local function BillLoop()
        while wait() do
        local pos = math.floor((BillboardGui.Parent.Position - BillboardGui.Parent.Position).magnitude)
            TextLabel.Text = text.."   Studs : "..tostring(pos)

        end
    end
        Loop = game:GetService("RunService").RenderStepped:Connect(BillLoop)
end
setBill("商店", game.Workspace["测试部件"])