Why is that script not working ? And Why is the script not calling correctly the event ?

Tool Script (LocalScript) -- Tool is in StarterPack Tool.Name is the ore name

tool = script.Parent
ReplicatedStorage = game.ReplicatedStorage
dropEvent = ReplicatedStorage:WaitForChild("dropEvent")


function drop()
	local oreName = tool.Name
	local Player = tool.Parent.Name
	print("Calling Event")
	dropEvent:FireServer(Player, tool.Name, tool)
	tool.Number.Value = tool.Number.Value - 1
	verif()
end

function verif()
	if tool.Number.Value <= 0 then
		tool:Destroy()
	end
	
end


tool.Activated:Connect(drop)

Event Script (server Script) -- In ServerScriptService

ReplicatedStorage = game.ReplicatedStorage
dropEvent = Instance.new("RemoteEvent", ReplicatedStorage)
dropEvent.Name = "dropEvent"


local function drop(Player, oreName, tool)
	print("Event called")
	print(Player, " ", oreName, " ", tool)
	local Ore = game.ReplicatedStorage.Ores:FindFirstChild(oreName)
	local newOre = Ore:Clone()
	newOre.Owner.Value = Player
	newOre.Name = "Ore"
	newOre.Ore.Value = oreName
	newOre.Parent = workspace.Loots
	
end


dropEvent.OnServerEvent:Connect(drop)

-- Studio Test Output -- Calling Event Event called alancretel1999 alancretel1999 Stone 13:14:15.214 - ServerScriptService.lootOre:10: attempt to index local 'Ore' (a nil value) 13:14:15.216 - Stack Begin 13:14:15.218 - Script 'ServerScriptService.lootOre', Line 10 13:14:15.219 - Stack End

-- Studio Team Test Output -- Calling Event

Thanks for your help, I'm new inLocalScript and custom events

When you fired the server event, you fired the player along with it. You don't need to do this, it'll add 2 players as the arguments of the function on the server script.

So instead of doing this:

dropEvent:FireServer(Player, tool.Name, tool)

Drop out the "Player" variable. It'll fix the arguments. Player will always be an added argument to this.