I'm trying to make a simple fire ball script, but I can't use LocalPlayer?

I can't use LocalPlayer in a serverscript but I don't know how else to reference the player. I need help.

Here's the code:

local UIS = game:GetService("UserInputService")
local RS = game:GetService("ReplicatedStorage")
local RE = RS.SimpleFireBall

RE.onServerEvent:Connect(function(simpleFireBall)
	local fireBall = Instance.new("Part", workspace)
	fireBall.Anchored = false
	fireBall.Size = Vector3.new(2,2,2)
	fireBall.Shape = "Block"
	local plr = game.Players.LocalPlayer
	local char = plr.Character
	local hum = char.Humanoid
	fireBall.Orientation = char.LeftArm.Orientation
	fireBall.CFrame = char.LeftArm.CFrame * CFrame.new(1,3,0)
end)

Thanks. Also here's the localscript:

local UIS = game:GetService("UserInputService")
local RS = game:GetService("ReplicatedStorage")
local RE = RS.SimpleFireBall

UIS.InputBegan:Connect(function(input, gameProcessedEvent)
	if input.KeyCode == Enum.KeyCode.Z then
		RE:FireServer()
	end
end)

every time you fire a server (in a local script) the localplayer instance comes with the arguments in the onserverevent function. example:

--localscript
local event = game.ReplicatedStorage.Event
event:FireServer(argument)

--server
local event = game.ReplicatedStorage.Event
event.OnServerEvent:Connect(function(player,argument)
	if player ~= nil then
		print("got player! woo!")
	end
end)

hope that helped