Attempt to index nil with lookVector?

Edited with working code!

Local Script:

--// Services \\--
local ReplicatedStorage = game:GetService("ReplicatedStorage")

--// Variables \\--
local Remote = ReplicatedStorage.FireBallEvent
local PlayerScripts = script.Parent
local Player = PlayerScripts.Parent
local Mouse = Player:GetMouse()
local Char = Player.Character or Player.CharacterAdded:Wait()

local Debounce = false
Mouse.Button1Down:Connect(function() 

local CharRP = Char.HumanoidRootPart
    if(Debounce == false) then
        Debounce = true
        Remote:FireServer((Mouse.Hit.Position - CharRP.Position).unit)<-- pass to the server the mockup MouseLookvetor
        Debounce  = false
    end
end)

Server Side:

--// Services \\--
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local Debris = game:GetService('Debris')

--// Variables \\--
local Remote = ReplicatedStorage.FireBallEvent
local FireBall = ReplicatedStorage.FireBall
local player = game.Workspace:FindFirstChild(game.Workspace.PlayerName.Value)

--// Settings \\--
local Damage = 20

local ServerDebounces = {}

Remote.OnServerEvent:Connect(function(plr,MouseLV)
 if not ServerDebounces[plr] then
  ServerDebounces[plr] = true
  local Char = plr.Character or plr.CharacterAdded:Wait()

  local FireBallClone = FireBall:Clone()
  FireBallClone.CFrame = Char.HumanoidRootPart.CFrame * CFrame.new(0,0,-3)
  FireBallClone.Parent = workspace

  local BodyVelocity = Instance.new("BodyVelocity")
  BodyVelocity.MaxForce = Vector3.new(math.huge, math.huge, math.huge)
  BodyVelocity.Velocity = MouseLV ----- This is the problem line
  BodyVelocity.Parent = FireBallClone

  spawn(function()
   for i = 0, 1000 do
    wait()
    FireBallClone.Size = FireBallClone.Size + Vector3.new(0.07,0.07,0.07)
    BodyVelocity.Velocity = MouseLV*i
   end
  end)
  ServerDebounces[plr] = false
  Debris:AddItem(FireBallClone, 10)

  local Debounce = true
  FireBallClone.Touched:Connect(function(h)
   if h.Parent:FindFirstChild('Humanoid') and h.Parent.Name ~= plr.Name and Debounce then   
    Debounce = false   
    local Enemy = h.Parent.Humanoid
    Enemy:TakeDamage(Damage)
    FireBallClone.Transparency = 1
    BodyVelocity:Destroy()
    wait(1)
    FireBallClone:Destroy()
    wait(3)
    Debounce = true
   end
  end)
 end
end)

as you can see i've changed the code a little bit as passing the Mouse object to the server didnt work, BUT sending the mockup Lookvector does! thats what this example does. It incorporates the HumanoidRootPart.Position and the Mouse.Hit.Position from the local script so it keeps the server script happy. I haven't fully tested the code but it works as far as firing the fireball from your chest area and it slowly moves away then speeds up.

I think the problem you ran into is the ServerEvent object allways returns the player who fired it and the server cannot use the Mouse object from the clients/players. But we can make a workaround as i've done in the scripts above by passing the lookvector as a Vector3 Value.

Might need to refine when defining the players root part on the server side script by passing the character as a varable or something.

Hope this helps!

p.s looks like a cool idea you have going on here! :D

You’re trying to send the player’s mouse, an object that only exists on the client, to the server, meaning it will come up as nil.

In the LocalScript sided, where you made the :FireServer(), please replace it to this:

local Mouse = game.Players.LocalPlayer:GetMouse()
game.ReplicatedStorage.FireBallEvent:FireServer(Mouse.lookVector)

And then replace in line 27 of the server script to:

BodyVelocity.Velocity = Mouse

So you got the lookVector via localscript, and you won't get the lookVector in server script, because mouse is only seen from client.