how to make count the number of items in a frame ?

output:01:02:56.481 - Players.Player1.PlayerGui.ScreenGui.LocalScript:78: attempt to get length of upvalue 'a' (a userdata value)

local gui = script.Parent
local guii = gui.Frame
local button1 = gui.button
local Button = script.Parent.TextButton
local slots = gui.Frame
local m = script
local pack
local ply = gui.Parent.Parent
local selected
local storage = gui.storage.Value
local s = 0
local a = slots:GetChildren()


button1.MouseButton1Up:connect (function()
    a = Instance.new("TextLabel")
    a.Parent = game.Lighting:findFirstChild(ply.Name.."sPack")

if s == 3 then
    print (s)

end
end)


y = 0.03
x = 0.01
maxX = 0.9
maxY = 0.9
-- Y + 60
-- X + 40
function getMyPack()
pack = game.Lighting:findFirstChild(ply.Name.."sPack")
end

function set(button)
if selected ~= nil then selected=nil selected["Selected"]:Remove() end
wait(.01)
selected=button
p=m.Bevel:Clone()
p.Parent=button
p.Name="Selected"
end

function removeOldOutOfTheWay(location)
for index, child in pairs(location) do
if child.Name == "TextButton" then
child:Remove()
y = 0.03
x = 0.01
end
end
end

function New()
removeOldOutOfTheWay(slots:getChildren())
local lo = pack:getChildren()
for index, child in pairs(lo) do
neww = Button:Clone()
neww.Position = UDim2.new(x, 0, y, 0)
neww.Text = child.Name
neww.Parent = slots
neww.MouseButton1Down:connect(function() set(neww) end)
x = x + 0.2
if x > 0.9 then
y = y +0.25
x = 0.01
end
end
end


getMyPack()
New()

pack.ChildAdded:connect(function()
New()
for i = 1,#a do -------------------------------------------Problem
    a[i] = s+1
end

end)

pack.ChildRemoved:connect(function()
New()
end)