instance for network manager

I made a turn based multiplayer server for mobile but having a compile error. Does anyone know how to solve this?

[SyncVar(hook = "OnTurnChange")] public bool isTurn = false;

 [SyncVar(hook = "UpdateTimeDisplay")]
 public float time = 100;

 public PlayerController controller;

 [SyncVar]
 public bool ready = false;

 // Use this for initialization
 void Start()
 {
     controller.OnPlayerInput += OnPlayerInput;
 }

 // Update is called once per frame
 [Server]
 void Update()
 {
     if (isTurn)
     {
         time -= Time.deltaTime;
         if (time <= 0)
         {
             NetworkManager.Instance.AlterTurns();
         }
     }
 }

 public override void OnStartClient()
 {
     DontDestroyOnLoad(this);

     base.OnStartClient();
     Debug.Log("Client Network Player start");
     StartPlayer();

     NetworkManager.Instance.RegisterNetworkPlayer(this);
 }

 public override void OnStartLocalPlayer()
 {
     base.OnStartLocalPlayer();
     controller.SetupLocalPlayer();
 }

 [Server]
 public void StartPlayer()
 {
     ready = true;
 }

 public void StartGame()
 {
     TurnStart();
 }

 [Server]
 public void TurnStart()
 {
     isTurn = true;
     time = 90;
     RpcTurnStart();
 }

 [ClientRpc]
 void RpcTurnStart()
 {
     controller.TurnStart();
 }

 [Server]
 public void TurnEnd()
 {
     isTurn = false;
     RpcTurnEnd();
 }

 [ClientRpc]
 void RpcTurnEnd()
 {
     controller.TurnEnd();
 }

 public override void OnNetworkDestroy()
 {
     base.OnNetworkDestroy();
     NetworkManager.instance.DeregisterNetworkPlayer(this);
 }

 public void OnTurnChange(bool turn)
 {
     if (isLocalPlayer)
     {
         //play turn sound 
     }
 }

 public void UpdateScore(int score)
 {
     Debug.Log("score:" + score);
 }

 void OnPlayerInput(PlayerAction action, float amount)
 {
     if (action == PlayerAction.SHOOT)
     {
         CmdOnPlayerInput(action, amount);
     }
 }

 [Command]
 void CmdOnPlayerInput(PlayerAction action, float amount)
 {
     //Shoot bullets

     //Update score
     NetworkManager.instance.UpdateScore(amount);
 }

 public void UpdateTimeDisplay(float curtime)
 {
     GameObject timerText = GameObject.FindWithTag("Timer");
     Text timer = timerText.GetComponent<Text>();
     timer.text = Mathf.Round(curtime).ToString();
 }

It shows an error of Assets\Scripts\NetworkManager.cs(51,24): error CS0117: 'NetworkManager' does not contain a definition for 'Instance'

Help me pls~