A connection has already been set as ready. There can only be one.

This is code of NetworkManager:
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Networking;
using UnityEngine.Networking.Match;
using UnityEngine.UI;
using UnityEngine.Networking.NetworkSystem;
using UnityEngine.SceneManagement;

public class LobbyManager : NetworkManager {

	public Transform spawnPosition;
	public Text tct;

	public int curPlayer;

	void Start () {
	
		curPlayer = GameObject.Find ("SelectCharVar").GetComponent<SelectCharacter> ().option;
	}
		

	/// Copied from Unity's original NetworkManager script except where noted
	public override void OnClientConnect(NetworkConnection conn)
	{
		/// ***
		/// Can't directly send an int variable to 'addPlayer()' so you have to use a message service...
		IntegerMessage msg = new IntegerMessage(curPlayer);
		/// ***

		if (!clientLoadedScene)
		{
			// Ready/AddPlayer is usually triggered by a scene load completing. if no scene was loaded, then Ready/AddPlayer it here instead.
			ClientScene.Ready(conn);
			if (autoCreatePlayer)
			{
				///***
				/// This is changed - the original calls a differnet version of addPlayer
				/// this calls a version that allows a message to be sent
				ClientScene.AddPlayer(conn, 0, msg);
			}
		}

	}

	/// Copied from Unity's original NetworkManager 'OnServerAddPlayerInternal' script except where noted
	/// Since OnServerAddPlayer calls OnServerAddPlayerInternal and needs to pass the message - just add it all into one.
	public override void OnServerAddPlayer(NetworkConnection conn, short playerControllerId, NetworkReader extraMessageReader)
	{
		/// *** additions
		/// I skipped all the debug messages...
		/// This is added to recieve the message from addPlayer()...
		int id = 1;

		if (extraMessageReader != null)
		{
			IntegerMessage i = extraMessageReader.ReadMessage<IntegerMessage>();
			id = i.value;
		}

		/// using the sent message - pick the correct prefab
		GameObject playerPrefab = spawnPrefabs[id];
		/// *** end of additions

		GameObject player;
		Transform startPos = GetStartPosition();
		if (startPos != null)
		{
			player = (GameObject)Instantiate(playerPrefab, startPos.position, startPos.rotation);
		}
		else
		{
			player = (GameObject)Instantiate(playerPrefab, Vector3.zero, Quaternion.identity);
		}
			
			player.name = GameObject.Find ("SelectCharVar").GetComponent<SelectCharacter> ().nickname;
			NetworkServer.AddPlayerForConnection(conn, player, playerControllerId);
	}

}

and it shows me the error:
A connection has already been set as ready. There can only be one.
UnityEngine.Networking.NetworkIdentity:UNetStaticUpdate()

probe of everything. Please help

public override void OnClientSceneChanged(NetworkConnection conn)
{
//base.OnClientSceneChanged(conn);

        //by overriding this function and commenting the base we are removing the functionality of this function 
        //so we dont have conflict with  OnClientConnect

    }

add this to your NetworkManager