How do I properly deal with Internal_CloneSingle can only be called from the main thread

So I've developed my own network protocol built upon the already existing protocol, TCP.

I'm now finally implementing it to my game which is not a complex game at all. All it's going to do, is spawn in the player when the client connects to the server.

The issue I'm facing now is when I try to call Instantiate(); it throws the exception:

 UnityEngine.UnityException: Internal_CloneSingle can only be called from the main thread.
 Constructors and field initializers will be executed from the loading thread when loading a scene.
 Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function.
   at (wrapper managed-to-native) UnityEngine.Object.Internal_CloneSingle(UnityEngine.Object)
   at UnityEngine.Object.Instantiate[T] (T original) [0x00017] in C:\buildslave\unity\build\Runtime\Export\UnityEngineObject.bindings.cs:275 
   at GameManager.SpawnPlayer (System.Int32 id, System.String username) [0x00023] in C:\Users\user\Documents\Astaria\Assets\Scripts\GameManager.cs:37 
   at ClientHandleData.SpawnPlayer (System.Byte[] data) [0x000a9] in C:\Users\user\Documents\Astaria\Assets\Scripts\ClientHandleData.cs:166 
   at ClientHandleData.HandlePacket (System.Byte[] data) [0x00031] in C:\Users\user\Documents\Astaria\Assets\Scripts\ClientHandleData.cs:95 
   at ClientHandleData.HandleData (System.Byte[] data) [0x000ef] in C:\Users\user\Documents\Astaria\Assets\Scripts\ClientHandleData.cs:66 
   at GameNet.OnReceive (System.IAsyncResult ar) [0x0003c] in C:\Users\user\Documents\Astaria\Assets\Scripts\GameNet.cs:62 
 UnityEngine.Debug:LogError(Object)
 GameNet:OnReceive(IAsyncResult) (at Assets/Scripts/GameNet.cs:70)
 System.Threading._ThreadPoolWaitCallback:PerformWaitCallback()

I have this empty object in my scene called GameManager in which I've attached the GameManager script to and it looks like this

 public class GameManager : MonoBehaviour
 {
     public static GameManager instance;
     public static Dictionary<int, Client> players = new Dictionary<int, Client>();
     public GameObject localPlayerPrefab;
     public GameObject playerPrefab;
 
     private void Awake()
     {
         if (instance == null)
         {
             instance = this;
         }
         else if (instance != this)
         {
             Destroy(this);
         }
     }
 
     public void SpawnPlayer(int id, string username)
     {
 
         //Instantiate can only be called from the main thread..
         GameObject _player;
         if (id == Client.instance.clientId)
             _player = Instantiate(localPlayerPrefab);
         else
             _player = Instantiate(playerPrefab);
 
         players.Add(id, _player.GetComponent<Client>());
         Debug.Log($"PLAYER WITH THE ID: {id} HAS CONNECTED!");
     }
 }

And that function gets called from my ClientHandleData.cs

 private void SpawnPlayer(byte[] data)
 {
     Packet packet = new Packet();
     packet.WriteBytes(data);
     var clientId = packet.ReadInt();
     var username = packet.ReadString();
     
     //Spawn the connected player on my screen
     GameManager.instance.SpawnPlayer(clientId, username);
 }

Which is defined in a dictionary at the top of the class

 public delegate void Packet_(byte[] data);
 private Dictionary<byte, Packet_> packets;
 public void InitializePackets()
 {
     Debug.Log("Initializing Network Packets..");
     packets = new Dictionary<byte, Packet_>();
     packets.Add((byte)GameOpCodes.PlayerData, SpawnPlayer);
 
 }

The function InitializePackets gets called from Client.cs which looks like this

 public class Client : MonoBehaviour
 {
     public static ClientHandleData HandleData = new ClientHandleData();
     public static GameNet client;
     public string username;
 
     public static Client instance;
 
     private void Awake()
     {
         instance = this;
     }
 
     private void Start()
     {
         HandleData.InitializePackets();
         client = new GameNet();
         client.ConnectToServer();
     }
 
 }

And the way the Client receives data is from GameNet.cs which has a socket callback that says OnReceive

Which then runs this

 Client.HandleData.HandleData(myBytes);

And that's how it's all tied together.

Why is it throwing the exception and how do I properly deal with it?