Why am I getting an error saying my function is nil?

I've been looking all over this script but I can't find why its saying my update() function is nil... This question is mostly just about the first "if then" statement and the update() function but I included the whole script to give better context. Let me know if you have any questions, and thanks for any help!

local BUTTON = script.Parent
local player = game.Players.LocalPlayer
local MFRAME = game.Players.LocalPlayer.PlayerGui.BackpackGui.MFrame
local IFRAME = game.Players.LocalPlayer.PlayerGui.BackpackGui.MFrame.IFrame
local OPEN = game.Players.LocalPlayer.PlayerGui.BackpackGui.Backpack:FindFirstChild("Open")
local mouse=player:GetMouse()
local RANGE = 15
------------------------------
wait(3)

local CURRENT = nil

local RANGE = 15

MFRAME.Visible = false
-------------------------------
if OPEN ~= nil then 
     update() print("yesss")
end


function removeExample()
    local player = game.Players.LocalPlayer
    local char = player.Character

    local ex_check = char:findFirstChild("BACKPACK_EXAMPLE")

    if ex_check ~= nil then
        ex_check:remove()
    end
end

function rangeCheck(mouse)
    local player = game.Players.LocalPlayer
    local char = player.Character
    local torso = char.Torso

    return (torso.Position - mouse.hit.p).magnitude < RANGE
end

function do_remove_all(parent)
    local children = parent:GetChildren()

    for index, child in pairs(children) do
        child:remove()
    end
end



function onGuiClick(part)
    CURRENT = part
    update()
end


function update()
    local player = game.Players.LocalPlayer
    local char = player.Character
    local pack = player:findFirstChild("Pack")
    local pack2 = player:findFirstChild("Pack2")
    local pack_tab = pack:GetChildren()
    local Ipack_tab = IFRAME:GetChildren()

    if CURRENT == nil then print ("current is nil")
        local x = 0
        local y = 0

        for index, child in pairs(pack_tab) do
            local CHOICE_GUI = MFRAME.TextButton:clone()
CHOICE_GUI.Size = UDim2.new(0, 100, 0, 20)
CHOICE_GUI.Transparency = .4
CHOICE_GUI.BackgroundColor3 = Color3.new()
            local new_gui = CHOICE_GUI 
            CURRENT = new_gui
            new_gui.Position = UDim2.new(0, x, 0, y)
            new_gui.Text = child.Name
            new_gui.TextColor3 = Color3.new(255,255,255)
            new_gui.Name = child.Name
            new_gui.MouseButton1Down:connect(function() onGuiClick(child) end)
            new_gui.Parent = IFRAME
            new_gui.Visible = true
            local CHILD = pack:FindFirstChild(child.Name)
            CHILD.Parent = pack2

            y = y + 20

            if y > 240 then
                y = 0
                x = x + 100

            end

        end

        --cleanGui()
    end
    end

function do_remove_all(parent)
    local children = parent:GetChildren()

    for index, child in pairs(children) do
        child:remove()
    end
end

function rangeCheck(mouse)
    local char = player.Character
    local torso = char.Torso

    return (torso.Position - mouse.hit.p).magnitude < RANGE
end

function onMouseButton1Down()
    local pack2 = player:findFirstChild("Pack2")
local pack_tab = pack2:GetChildren()
local partname = BUTTON.Text
local part = pack2:FindFirstChild(partname)
    local example = part:Clone()
    local char = player.Character
    MFRAME.Visible = false

    mouse.Button1Down:connect(function() onButton1Down(mouse) end)
    mouse.Move:connect(function() onMouseMove(mouse) end)
    mouse.KeyDown:connect(function(key) onKeyDown(key, mouse) end)

end

function removeExample()
    local player = game.Players.LocalPlayer
    local char = player.Character

    local ex_check = char:findFirstChild("BACKPACK_EXAMPLE")

    if ex_check ~= nil then
        ex_check:remove()
    end
end


function onMouseMove(mouse)
    local player = game.Players.LocalPlayer
    local char = player.Character
    local torso = char.Torso
    local pack2 = player:findFirstChild("Pack2")
local pack_tab = pack2:GetChildren()
local partname = BUTTON.Text
local part = pack2:FindFirstChild(partname)

local CURRENT = part
    removeExample()


    if CURRENT ~= nil then
        if rangeCheck(mouse) then

            local example = CURRENT:clone()
            example.Name = "BACKPACK_EXAMPLE"
            example.Locked = true
            example.Transparency = 0.5
            example.Parent = char
            example.Position = mouse.hit.p
            do_remove_all(example)

        end
    end
end

function onButton1Down(mouse)
    local pack2 = player:findFirstChild("Pack2")
local pack_tab = pack2:GetChildren()
local partname = BUTTON.Text
local part = pack2:FindFirstChild(partname)
local player = game.Players.LocalPlayer
local char = player.Character
local example = char:FindFirstChild("example")
local CURRENT = part

    MFRAME.Visible = true

    if CURRENT ~= nil then
        if rangeCheck(mouse) then
            removeExample()
            print("clicked")

            CURRENT.Parent = game.Workspace
            CURRENT.Locked = false
            CURRENT.Anchored = false
            CURRENT.Position = mouse.hit.p
            CURRENT = nil
            --do_remove_all(example)
            --cleanGui()
            script.Parent.Visible = false
            script.Parent:Destroy() print("buttonremoved")
            update() print("primaryupdate")


    end
    end
    end

function onKeyDown(key, mouse)
    if key == "r" then
        removeExample()
        local CURRENT = nil
    elseif key == "e" then
        local player = game.Players.LocalPlayer
        local pack = player:findFirstChild("Pack")
        local pack_tab = pack:GetChildren()

        for index, child in pairs(pack_tab) do
            local CURRENT = child
            onButton1Down(mouse)
            wait(0.1)
        end

        local CURRENT = nil
    end
end


BUTTON.MouseButton1Down:connect(onMouseButton1Down)