It says Attachment is not a valid member of part?

As TheeDeathCaster said, the script loaded before the attachment did. Add local attachment = FireBallClone:WaitForChild("Attachment") somewhere and replace FireBallClone.Attachment on the other lines with attachment. And here's your script that should be fixed by now.

-- Variables --
local Remote = ReplicatedStorage.FireBallEvent
local FireBall = ReplicatedStorage.FireBall

Remote.OnServerEvent:Connect(function(plr, Mouse)
 if not ServerDebounces[plr] then
  ServerDebounces[plr] = true

-- Settings --  

  local Char = plr.Character or plr.CharacterAdded:Wait()

  local FireBallClone = FireBall:Clone()
local attachment = FireBallClone:WaitForChild("Attachment")

 local Enemy = Hum.Parent.Humanoid
    Enemy:TakeDamage(Damage)
    BodyVelocity:Destroy()
    attachment.Effect.Speed = NumberRange.new(8,8)
    wait(1)
    attachment.Effect.Enabled = false
    attachment.SmokeEffect.Enabled = false
    wait(1)
    FireBallClone:Destroy()

Put these two in StarterPack:

For pressing key: (Local Script)

local Player = game.Players.LocalPlayer
local Mouse = Player:GetMouse()
local RemoteEvent = game.Workspace.Fired
local UIS = game:GetService("UserInputService")

local Debounce = false

UIS.InputBegan:Connect(function(Input, gameProcessed)
    if Input.KeyCode == Enum.KeyCode.F and gameProcessed == false then
        if Debounce == true then return end
        Debounce = true
        RemoteEvent:FireServer()
        wait(0.5) -- Change the interval between each fireball
        Debounce = false
    end
end)

Fireball creation (Normal Script)

game.Workspace.Fired.OnServerEvent:Connect(function(Player, Debounce)
    local AlreadyTouched = false
    local Character = Player.Character or Player.CharacterAdded:wait()
    local animation = Instance.new("Animation")
    animation.AnimationId = "rbxassetid://846744780"
    animation = Character.Humanoid:LoadAnimation(animation)
    animation:Play() ----- Active animation
    wait(0.5)
    local Fireball = Instance.new("Part")
    local Fire = Instance.new("ParticleEmitter",Fireball)
    Fireball.Name = "Fireball"
    Fireball.Shape = Enum.PartType.Ball
    Fireball.Size = Vector3.new(2,2,2)
    Fireball.Material = Enum.Material.Neon
    Fireball.BrickColor = BrickColor.new("Deep orange")
    Fireball.CanCollide = false
    Fire.LightEmission = NumberRange.new(0.35)
    Fire.LightInfluence = NumberRange.new(0)
    Fire.Texture = "rbxassetid://405886187"
    Fire.Lifetime = NumberRange.new(0.5)
    Fire.Rate = 20
    Fire.Speed = NumberRange.new(3)
    Fire.SpreadAngle = Vector2.new(15.15)
    Fireball.Parent = Character
    Fireball.CFrame = Character.HumanoidRootPart.CFrame*CFrame.new(0,1,-3)
    local BV = Instance.new("BodyVelocity")
    BV.MaxForce = Vector3.new(math.huge,math.huge,math.huge)
    BV.Velocity = Character.HumanoidRootPart.CFrame.lookVector*100
    BV.Parent = Fireball

    Fireball.Touched:Connect(function(Hit)
        local Humanoid = Hit.Parent:FindFirstChild("Humanoid")

        if Humanoid == nil then return end

        if AlreadyTouched == false then
            AlreadyTouched = true
            local debris = game:GetService("Debris")
                if Humanoid then 
                    local creatorTag = Instance.new("ObjectValue")
                    creatorTag.Value = Player
                    creatorTag.Name = "creator"
                    creatorTag.Parent = Humanoid
                    debris:AddItem(creatorTag, 1)
                end;
            if Humanoid.Parent == Character then
                Humanoid.Health = Humanoid.Health - 0
            else
                Humanoid.Health = Humanoid.Health - Humanoid.MaxHealth/2
                Fireball:Destroy()
            end
        end
    end)
    wait(3)
    Fireball:Destroy()
end)

And finally make a RemoteEvent in Workspace named "Fired"