How do I set non-player object client authority?

Hello All,

I've been playing around with this for sometime and can't seem to figure it out. I've created a simplified version of a game I'm making to understand the code better. What I would like to do is have multiple people be able to move around a non-player object. In my current set up I have a player object (red cube) and a non-player object (blue cube). With the script I have now I can move the player object on the host/client side and have it update on the host/client side. When I try to move the blue cube I get the following error:

AssignClientAuthority for Cube (UnityEngine.GameObject) owner cannot be null. Use RemoveClientAuthority() instead. ​Here is the code I have attached to the non-player object (blue cube):

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine.Networking;
  
 public class DnDNonPlayer : NetworkBehaviour
 {
     GameObject cube;
  
     // Use this for initialization
     void Start ()
     {
         cube = gameObject;
     }
  
     void OnMouseDown ()
     {
         if (isLocalPlayer)
             return;
         CmdSetAuthority(cube.GetComponent<NetworkIdentity>());
     }
  
     [Command]
     void CmdSetAuthority(NetworkIdentity ObjID)
     {
         ObjID.AssignClientAuthority(connectionToClient);
     }
  
     void OnMouseDrag ()
     {
  
         if (!isLocalPlayer)
             return;
  
         Ray Ray = Camera.main.ScreenPointToRay (Input.mousePosition);
         RaycastHit hitInfo;
  
         if (Physics.Raycast(Ray, out hitInfo))
         {
  
             Vector3 drag = new Vector3 (hitInfo.point.x, 0.5f, hitInfo.point.z);
             transform.position = drag;
         }
     }
  
     void OnMouseUp()
  
     {
         if (!isLocalPlayer)
             return;
  
         Ray Ray2 = Camera.main.ScreenPointToRay (Input.mousePosition);
         RaycastHit hitInfo2;
  
         Physics.Raycast (Ray2, out hitInfo2);
  
         Vector3 objPosition = new Vector3 (hitInfo2.point.x, 0.5f, hitInfo2.point.z);
         transform.position = objPosition;
     }
 }

I also expect that I will have to remove client authority as well but i clearly haven't gotten that far yet. Also note I have next to zero experience programing and just started learning. Most of the stuff here is hacked together based on what I've found here on the forum and other places online. Thanks and I appreciate any help.