upvalues

function Unscaffoldise(scaffolding)
    local parts = scaffolding:GetChildren()
    table.sort(parts, function (a, b) if a.Position.Y > b.Position.Y then return true else return false end end)
    local decks = {}
    local poles = {}
    for i = 1, #parts do
        if parts[i].Name == "Pole" then
            table.insert(poles, parts[i])
            elseif parts[i].Name == "Deck" then
            table.insert(decks, parts[i])
        end
    end

    local Poles_per_level = 0
    local height = 0
    for i = 1, #poles do
        if height == 0 then
            height = math.floor(poles[i].Position.Y)
            Poles_per_level = Poles_per_level + 1
            elseif height ~= math.floor(poles[i].Position.Y) then
            break
            else
            Poles_per_level = Poles_per_level + 1
        end
    end

    local levels = (#poles / Poles_per_level) - 1

    local OldCFrame = decks[1].CFrame
    decks[1].Size = decks[1].Size + Vector3.new(0.01, 0.01, 0.01)
    decks[1].CFrame = OldCFrame

    for i = 1, Poles_per_level do
        delay(0, function ()
            for y = 6, 0, -2 do
            wait(0.2)
                if y == 0 then
                    poles[i]:Destroy()
                else
                    local NewCFrame = poles[i].CFrame + Vector3.new(0, -1, 0)
                    poles[i].Size = Vector3.new(2, y, 2)
                    poles[i].CFrame = NewCFrame
                end
            end
        end)
    end

    wait(0.2 * 4)

    local polesdeleted = Poles_per_level
    for level = 1, levels do
        for i = polesdeleted + 1, polesdeleted + Poles_per_level do
            delay(0, function ()
                for y = 6, 0, -2 do
                    wait(0.2)
                    if y == 0 then
                        poles[i]:Destroy()
                    else
                        local NewCFrame = poles[i].CFrame + Vector3.new(0, -1, 0) --attempt to index upvalue 'poles' (a nil value)
                        poles[i].Size = Vector3.new(2, y, 2)
                        poles[i].CFrame = NewCFrame
                    end
                end
            end)
        end
        wait(0.2 * 4)
        polesdeleted = polesdeleted + Poles_per_level
        if level ~= levels then
            TweenTo(decks[1], decks[2].Position, 0.5, "Bounce") --attempt to index field '?' (a nil value)
            decks[2]:Destroy()
            table.remove(decks, 2)
        else
            TweenTo(decks[1], decks[1].Position + Vector3.new(0, -8, 0), 0.5, "Bounce")
        end
    end
    decks[1]:Destroy()
end

--This bit calls the function

scaffs = {}
for n, a in pairs(game.Workspace.Model:GetChildren()) do
    delay(0, function ()
        print(n)
        a.Parent = Instance.new("Model", a.Parent)
        scaffs[n] = Scaffoldise(a.Parent, a.Size, Instance.new("Model", game.Workspace))
        Build(a.Parent)
        Unscaffoldise(scaffs[n]) --everything works except this
    end)
end