Mirror could not be found in scripts but loaded in project

Question

My environment Unity 2021.2.11f1 using Mirror 57.0.0

Like a good boy, i just imported Mirror to my project, create a little network manager and it works, now i wanted to create an override from a Network Manager and create my own but, just typong "using Mirror" in scripts, there are already errors so when i check the problem tab in VS Code i got these errors

The type or namespace name 'NetworkManager' could not be found (are you missing a using directive or an assembly reference?) [Assembly-CSharp]

what i did was simply

using UnityEngine;
using UnityEngine.SceneManagement;
using Mirror;

public class MyNetworkManager : NetworkManager

What am i lacking here? Like what i said the Mirror is working if i just Add a network manager and network manager HUD in a game object, i ran and tested aand see that Mirror indeed works, just the reference in the code is missing or not found.