Attempt to index nil with CFrame?

I have a script that I want to make a projectile not move for 2 seconds and start moving again afterwards, but I get the "Attempt to index nil with CFrame" error. Here is my code:

local bullet = game.ReplicatedStorage["Cyborg Assets"].ball
local head = script.Parent.Parent:FindFirstChild("Head")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local event = Instance.new("RemoteEvent", ReplicatedStorage)
event.Name = "Cyborg SP3"
local debounce = false
--local sound = Instance.new("Sound")
--sound.Parent = head
--sound.SoundId = "rbxassetid://1382489817"
--sound.Volume = 2
--sound.Playing = false
--local sound2 = Instance.new("Sound")
--sound2.Parent = head
--sound2.SoundId = "rbxassetid://1382489817"
--sound2.Volume = 2
--sound2.Playing = false

local function Hurt(Part)
        if debounce == false then
    	    if Part.Parent:FindFirstChild("Humanoid") then
				Part.Parent.Humanoid.Health = Part.Parent.Humanoid.Health - 75
				Part.Parent.Humanoid.WalkSpeed = 8
				debounce = true
				wait(2)
				Part.Parent.Humanoid.WalkSpeed = 16
				debounce = false
			end
		end
end

local function shootbullet()
    local debounce = false
    local Fireball = bullet:Clone()
	print("remoteEvent recieved")
	--sound.Playing = true
	Fireball.BrickColor = BrickColor.new("Institutional white")
    Fireball.Transparency = 0
    Fireball.TopSurface = 0
    Fireball.BottomSurface = 0
    Fireball.CanCollide = false
    Fireball.CFrame = head.CFrame * CFrame.new(0,0,-15)
    Fireball.Parent = game.Workspace
	wait(2)
	Fireball.CFrame = head.CFrame * CFrame.new(0,0,-15)
	--sound2.Playing = true
    local BodyVelocity = Instance.new("BodyVelocity")
    BodyVelocity.Parent = Fireball
	BodyVelocity.maxForce = Vector3.new(math.huge, math.huge, math.huge)
	BodyVelocity.velocity = head.CFrame.lookVector*75
	Fireball.Touched:connect(Hurt)
	wait(10)
	Fireball:Destroy()
end
event.OnServerEvent:connect(shootbullet)

when your changing a cframe you must add CFrame.New try doing this in line 41

Fireball.CFrame = CFrame.New(head.CFrame * CFrame.new(0,0,-15))

if that doesnt work then

Fireball.CFrame = CFrame.New(head.CFrame * Vector3.new(0,0,-15))