Why is this ammo GUI script not working?

local tool = script.Parent
local player = game:GetService("Players").LocalPlayer
local ammo = 8
local isReload = false
local mouse = game.Players.LocalPlayer:GetMouse()
local maxAmmo = 8
local ammoGUI = player.PlayerGui:FindFirstChild("Ammo")
repeat wait() until ammoGUI
local GUI = ammoGUI.Background.Frame
repeat wait() until GUI


function reload(amount) --function to avoid retyping
    tool.Name = "Reloading"
    isReload = true
    mouse.Icon = "rbxgameasset://Images/GunWaitCursor"
    wait(2)
    ammo = amount
    GUI.Ammo.Text = ammo
    --tool.Name = "Pistol " .. tostring(ammo)
    isReload = false
    mouse.Icon = "rbxgameasset://Images/GunCursor"
end


tool.Equipped:connect(function(mouse)
    print("Pistol Equipped")
    mouse.Icon = "rbxgameasset://Images/GunCursor"
    GUI.MaxAmmo.Text = maxAmmo
    game:GetService('UserInputService').InputBegan:connect(function(input, gameprocessed)
        if input.KeyCode == Enum.KeyCode.R then
            reload(8)
        end
    end)
    mouse.Button1Down:connect(function()
    print("Mouse Clicked")
    if (not isReload) and ammo > 0 then --if Player is not reloading
            ammo = ammo - 1
            GUI.Ammo.Text = ammo
            --tool.Name = "Pistol " ..tostring(ammo)  
            local ray = Ray.new(tool.Handle.CFrame.p, (mouse.Hit.p - tool.Handle.CFrame.p).unit * 200)
            local part, position = workspace:FindPartOnRay(ray, player.Character, false, true)

            local beam = Instance.new("Part", workspace)
            beam.BrickColor = BrickColor.new("Gray")
            beam.FormFactor = "Custom"
            beam.Material = "Neon"
            beam.Transparency = 0
            beam.Anchored = true
            beam.Locked = true
            beam.CanCollide = false

            local light = Instance.new("PointLight", beam)
            light.Range = 15

            local distance = (tool.Handle.CFrame.p - position).magnitude
            beam.Size = Vector3.new(0.3, 0.3, distance)
            beam.CFrame = CFrame.new(tool.Handle.CFrame.p, position) * CFrame.new(0, 0, - distance / 2)

            game:GetService("Debris"):AddItem(beam, 0.1)

            if part then
                local humanoid = part.Parent:FindFirstChild("Humanoid")

                if not humanoid then
                    humanoid = part.Parent.Parent:FindFirstChild("Humanoid")
                end

                repeat wait() until humanoid
                local att = humanoid.Parent.Name
                local a = game.Players:FindFirstChild(att)  


                if humanoid then  
                    if player.TeamColor ~= a.TeamColor then --anti teamkill start
                        humanoid:TakeDamage(30)
                    end
                end

                if ammo == 0 then
                    reload(8)
                end
             end
        else
            reload(8)
        end
    end)
    tool.Unequipped:connect(function()
        print("Pistol Unequipped")
        mouse.Icon = "rbxgameasset://Images/ArrowCursor"
    end)
end)

It keeps saying the variable Ammo GUI is a nil value. It's placed inside of the starterGUI but I want the GUI to be different for every player.