Replacing existing prefab with asset id...

I'm looking at guides from DipperDino (DipperDino guide) on Mirror multiplayer and during development there was an error that it doesn't have. I rechecked everything many times and couldn't fix it. The error sounds like "replacing existing prefab with asset id", referring to objects in Rosouces.SpawnablePrefabs. I left the NetworkManagerLobby class code below.

 using Mirror;
 using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 
 public class NetworkManagerLobby : NetworkManager
 {
     [SerializeField] private int minPlayers = 2;
     [Scene] [SerializeField] private string menuScene = string.Empty;
 
     [Header("Room")]
     [SerializeField] private NetworkRoomPlayerLobby roomPlayerPrefab = null;
 
     [Header("Game")]
     [SerializeField] private NetworkGamePlayerLobby gamePlayerPrefab = null;
     [SerializeField] private GameObject playerSpawnSystem = null;
 
     public static event Action OnClientConnected;
     public static event Action OnClientDisconnected;
     public static event Action<NetworkConnection> OnServerReadied;
 
     public List<NetworkRoomPlayerLobby> RoomPlayers { get; } = new List<NetworkRoomPlayerLobby>();
     public List<NetworkGamePlayerLobby> GamePlayers { get; } = new List<NetworkGamePlayerLobby>();
     // HERE CAN BE SPECTATOR PLAYERS LIST FOR OWN SPECTATOR LOGIC (LIKE RoomPlayer -> GamePlayers BUT SpectatorPlayers -> GamePlayers
 
     public override void OnStartServer() => spawnPrefabs = Resources.LoadAll<GameObject>("SpawnablePrefabs").ToList();
 
     public override void OnStopServer()
     {
         RoomPlayers.Clear();
     }
 
     public override void OnStartClient()
     {
         var spawnablePrefabs = Resources.LoadAll<GameObject>("SpawnablePrefabs");
 
         foreach (var prefab in spawnablePrefabs)
         {
             ClientScene.RegisterPrefab(prefab);
         }
     }
 
     public override void OnClientConnect(NetworkConnection conn)
     {
         base.OnClientConnect(conn);
 
         OnClientConnected?.Invoke();
     }
 
     public override void OnClientDisconnect(NetworkConnection conn)
     {
         base.OnClientDisconnect(conn);
 
         OnClientDisconnected?.Invoke();
     }
 
     public override void OnServerConnect(NetworkConnection conn)
     {
         if (numPlayers >= maxConnections)
         {
             conn.Disconnect();
             return;
             //TODO; show err lobby is full message
         }
 
         if (SceneManager.GetActiveScene().path != menuScene)
         {
             conn.Disconnect();
             return;
             //TODO; show err scene message
         }
     }
 
     public override void OnServerDisconnect(NetworkConnection conn)
     {
         if (conn.identity != null)
         {
             var player = conn.identity.GetComponent<NetworkRoomPlayerLobby>();
 
             RoomPlayers.Remove(player);
 
             NotifyPlayersOfReadyState();
         }
 
         base.OnServerDisconnect(conn);
     }
 
     public override void OnServerAddPlayer(NetworkConnection conn)
     {
         if (SceneManager.GetActiveScene().path == menuScene)
         {
             bool isLeader = RoomPlayers.Count == 0;
 
             NetworkRoomPlayerLobby roomPlayerInstance = Instantiate(roomPlayerPrefab);
 
             roomPlayerInstance.IsLeader = isLeader;
 
             NetworkServer.AddPlayerForConnection(conn, roomPlayerInstance.gameObject);
         }
     }
 
     public void NotifyPlayersOfReadyState()
     {
         foreach (var player in RoomPlayers)
         {
             player.HandleReadyToStart(IsReadyToStart());
         }
     }
 
     private bool IsReadyToStart()
     {
         if (numPlayers < minPlayers) { return false; }
 
         foreach (var player in RoomPlayers) //Loop every player in the lobby
         {
             if (!player.IsReady) { return false; }
         }
 
         //########Loop every player in the lobby########
         //foreach (var player in RoomPlayers) 
         //{
         //    CODE HERE;
         //}
 
         return true;
     }
 
     public void StartGame()
     {
         if(SceneManager.GetActiveScene().path == menuScene)
         {
             if(!IsReadyToStart()) { return; }
 
             ServerChangeScene("Scene_Tests_01");
         }
     }
 
     public override void ServerChangeScene(string newSceneName)
     {
         // From menu to scene
         if (SceneManager.GetActiveScene().path == menuScene && newSceneName.StartsWith("Scene_Tests"))
         {
             for (int i = RoomPlayers.Count - 1; i >= 0; i--)
             {
                 var conn = RoomPlayers[i].connectionToClient;
                 var gameplayerInstance = Instantiate(gamePlayerPrefab);
                 gameplayerInstance.SetDisplayName(RoomPlayers[i].DisplayName);
 
                 NetworkServer.Destroy(conn.identity.gameObject);
 
                 NetworkServer.ReplacePlayerForConnection(conn, gameplayerInstance.gameObject, true);
             }
         }
 
         base.ServerChangeScene(newSceneName);
     }
 
     public override void OnServerSceneChanged(string sceneName)
     {
         if (sceneName.StartsWith("Scene_Tests"))
         {
             GameObject playerSpawnSystemInstance = Instantiate(playerSpawnSystem);
             NetworkServer.Spawn(playerSpawnSystemInstance);
         }
     }
 
     public override void OnServerReady(NetworkConnection conn)
     {
         base.OnServerReady(conn);
 
         OnServerReadied?.Invoke(conn);
     }
 }