Third Party MonoBehaviourPunCallbacks could not be loaded

Hi,
I'm following a tutorial on creating multiplayer with Photon Pun and am getting an error saying that the associated script can't be loaded.

This is my code:

Code (CSharp):

1. using Photon.Pun;
2. using Photon.Realtime;
3. using UnityEngine;
4.  
5. public class NetworkManager : MonoBehaviourPunCallbacks
6. {
7.     void Start()
8.     {
9.         ConnectToServer();
10.     }
11.  
12.     void ConnectToServer()
13.     {
14.         PhotonNetwork.ConnectUsingSettings();
15.         Debug.Log("Trying To Connect To Server...");
16.     }
17.  
18.     public override void OnConnectedToMaster()
19.     {
20.         Debug.Log("Connected To Server.");
21.         base.OnConnectedToMaster();
22.         RoomOptions roomOptions = new RoomOptions();
23.         roomOptions.MaxPlayers = 10;
24.         roomOptions.IsVisible = true;
25.         roomOptions.IsOpen = true;
26.      
27.         PhotonNetwork.JoinOrCreateRoom("Room 1", roomOptions, TypedLobby.Default);
28.     }
29.  
30.     public override void OnJoinedRoom()
31.     {
32.         Debug.Log("Joined a Room");
33.         base.OnJoinedRoom();
34.     }
35.  
36.     public override void OnPlayerEnteredRoom(Player newPlayer)
37.     {
38.         Debug.Log("A new player joined the room");
39.         base.OnPlayerEnteredRoom(newPlayer);
40.     }
41. }

Any help would be much appreciated.