An Object Reference is required to access non-static member in a Multiplayer game,'Object Reference Required to acces Non-Static Member' Networking

I'm fairly new to Unity, so I'm sorry if this question seems silly. I'm trying to work on separating instances of my game running on a server. In other words, I'd like to only be able to control the player in the active window, instead of both. To fix this I tried following some post somewhere but I'm pretty sure it's from unity 4 or earlier. I'm getting the error on line 44, 'if(networkView.isMine) etc.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Player : MonoBehaviour {
 
     public float speed = 10f;
 
     Rigidbody rigidbody; 
 
     void Start()
     {
         rigidbody = GetComponent<Rigidbody> ();
     }
 
     void OnSerializeNetworkView(BitStream stream, NetworkMessageInfo info)
     {
         Vector3 syncPosition = Vector3.zero;
         if (stream.isWriting) {
             syncPosition = rigidbody.position;
             stream.Serialize (ref syncPosition);
         } else 
         {
             stream.Serialize (ref syncPosition);
             rigidbody.position = syncPosition;
         }
     }
 
     void InputMovement()
     {
         if (Input.GetKey(KeyCode.W))
             rigidbody.MovePosition(rigidbody.position + Vector3.forward * speed * Time.deltaTime);
         if (Input.GetKey(KeyCode.S))
             rigidbody.MovePosition(rigidbody.position - Vector3.forward * speed * Time.deltaTime);
         if (Input.GetKey(KeyCode.A))
             rigidbody.MovePosition(rigidbody.position - Vector3.right * speed * Time.deltaTime);
         if (Input.GetKey(KeyCode.D))
             rigidbody.MovePosition(rigidbody.position + Vector3.right * speed * Time.deltaTime);
     }
     
     // Update is called once per frame
     void Update()
     {
         if (NetworkView.isMine)
         {
             InputMovement();
         }
     }
             
 
 }