How to add a tool in a backpack when you spawn?

I am trying to make a local script to add a tool in your back pack when I click a button. When I run my script, and spawn in, I get nothing when I click. Why? Here is my script.

local StarterGui = game:GetService('StarterGui') 
StarterGui:SetCoreGuiEnabled(Enum.CoreGuiType.Backpack, false)

local Player = game.Players.LocalPlayer
local Character = Player.Character or Player.CharacterAdded:Wait()
local Humanoid = Character.Humanoid

local WEAPON = Player.Backpack['M16'] 


local HumanoidRootPart = Character:WaitForChild('HumanoidRootPart')

local GotGun = false
local btn = game.StarterGui.ScreenGui.Frame.TextButton

btn.MouseButton1Click:Connect(function()



        if GotGun == false then
            Humanoid:EquipTool(WEAPON)
            GotGun = true
        end

end)
if Humanoid.Health == 0 then


    Humanoid:UnequipTools()

    GotGun = false
end