Attempt to index nil with 'leaderstats'??

I need help with this script. I am kinda confused about using functions and loops at the same time, don't know if it will work. Here is the script:

local ReplicatedStorage = game:GetService("ReplicatedStorage")

local remoteEvent = ReplicatedStorage:WaitForChild("Arrow")

local player = game.Players.LocalPlayer
local ServerStorage = game:GetService("ServerStorage")
local star = ServerStorage:WaitForChild("Star")
local cloneStar = star:Clone()
cloneStar.Parent = game.Workspace

local function onSpawnStar(cloneStar)
while true do
wait(.1)
local level = player.leaderstats.Level.Value
if game.Workspace.Checkpoints:FindFirstChild(level + 1) then
	local ls = game.Workspace.Checkpoints:FindFirstChild(level + 1)
		cloneStar:MoveTo(ls.Position + Vector3.new(0,10,0))
	end
	wait(1)
	if(player.leaderstats.Level.Value == script.Levels.Value)then
		cloneStar:Destroy()
end
	if(player.Character.Humanoid.Health == 0)then
		cloneStar:Destroy()
		end
	end
end

remoteEvent.OnServerEvent:Connect(onSpawnStar)

The error you're getting is because you're trying to use LocalPlayer on the server, it won't work because the server isn't associated with any of the clients so it'll only return nil (hence the error). LocalPlayer can only be obtained through LocalScripts as LocalScripts handle things on the client.

When firing RemoteEvents to the server, it automatically passes the player that fired it, so the first parameter on OnServerEvent is always going to be the player. At the moment with your script, your cloneStar parameter is actually the player that fired the RemoteEvent.

Remove your player variable and put "player" as your first parameter in your onSpawnStar function.