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)