[UNet] Instantiating over Network won't work

I have a multiplayer game here I need to instantiate cubes mid game, so they need to be visible on the server and clients. Currently when I try NetworkServer.Spawn it's displayed in my scene view but not in the server or client's game view, I am still able to jump on and run into the boxes though. I am pretty new to Unity so I don't really know why nothing its not showing up on my screen.

 public GameObject block;
 [Server]
 public void GenerateTerrain()
 {
                 Vector3 pos = new Vector3(x * 2 - 29, y * 2 - 198, z * 2 - 29);
                 GameObject cube = (GameObject)Instantiate(block, pos, Quaternion.identity);
                 NetworkServer.Spawn(cube);
 }

Calling script:

         if (isServer)
         {
             Debug.Log("Server");
             Spawner = GameObject.Find("Environment");
             Spawner.GetComponent<Spawner>().CmdGenerateTerrain();
         }

What I see in Unity: https://gyazo.com/61b45a3494b8d81d48f01220b0ca36c5