Why do i get this error in my muiltiplayer Game

This is my code below

 using UnityEngine;
 using System.Collections;
 using UnityEngine.Networking;
 using UnityEngine.UI;
 
 public class NetworkManager_Custom : NetworkManager {
 
 
     public void StartupHost()
     {
         SetPort();
         NetworkManager.singleton.StartHost();
     }
 
     public void JoinGame()
     {
         SetIPAddress();
         SetPort();
         NetworkManager.singleton.StartClient();
     }
 
     void SetIPAddress()
     {
         string ipAddress = GameObject.Find("InputFieldIPAddress").transform.FindChild("Text").GetComponent<Text>().text;
         NetworkManager.singleton.networkAddress = ipAddress;
     }
 
     void SetPort()
     {
         NetworkManager.singleton.networkPort = 7777;
     }
 
     void OnLevelWasLoaded(int level)
     {
         if (level == 0)
         {
             //SetupMenuSceneButtons();
             StartCoroutine(SetupMenuSceneButtons());
         }
 
         else
         {
             SetupOtherSceneButtons();
         }
     }
 
     IEnumerator SetupMenuSceneButtons()
     {
         yield return new WaitForSeconds(0.3f);
         GameObject.Find("ButtonStartHost").GetComponent<Button>().onClick.RemoveAllListeners();
         GameObject.Find("ButtonStartHost").GetComponent<Button>().onClick.AddListener(StartupHost);
 
         GameObject.Find("ButtonJoinGame").GetComponent<Button>().onClick.RemoveAllListeners();
         GameObject.Find("ButtonJoinGame").GetComponent<Button>().onClick.AddListener(JoinGame);
     }
 
     void SetupOtherSceneButtons()
     {
         GameObject.Find("ButtonDisconnect").GetComponent<Button>().onClick.RemoveAllListeners();
         GameObject.Find("ButtonDisconnect").GetComponent<Button>().onClick.AddListener(NetworkManager.singleton.StopHost);
     }
 
 
 }
 
 

and this is the error that i get

 Cannot open socket on ip {*} and port {7777}; check please your network, most probably port has been already occupied
 UnityEngine.Networking.NetworkManager:StartHost()
 NetworkManager_Custom:StartupHost() (at Assets/Scripts/Networking/NetworkManager_Custom.cs:12)
 UnityEngine.EventSystems.EventSystem:Update()

all help is appreciated