UNET Can't spawn object, network server not active...

I am trying to make a player selection, where the player chooses his player from ui buttons and then spawns it. The script is working perfectly for the host player, but not for the connected clients.

     [Command]
     public void CmdSpawnCharacter() {
         ClientScene.RegisterPrefab(PlayerPrefab); 
         var spawn = NetworkManager.singleton.GetStartPosition();
         GameObject newPlayer = (GameObject)Instantiate(PlayerPrefab, spawn.position, spawn.rotation);
         NetworkServer.Destroy(this.gameObject);
 
         if (this.playerControllerId == 0)
         {
             NetworkServer.SpawnWithClientAuthority(newPlayer, this.connectionToClient);
         }
         if (this.playerControllerId > 0)
         {
             NetworkServer.SpawnWithClientAuthority(newPlayer, this.connectionToServer);
         }
 
     }

The object is being spawned on the client side, but as soon as it is spawned i get the error

" SpawnObject for RoguePlayer(Clone) (UnityEngine.GameObject), NetworkServer is not active. Cannot spawn objects without an active server. UnityEngine.Networking.NetworkServer:Spawn(GameObject) CharacterSelect:CmdSpawnCharacter() (at Assets/Scripts/CharacterSelect.cs:75) UnityEngine.EventSystems.EventSystem:Update() "

I searched in a lot of places, and mostly all of them said to make the spawn function a [Command], but it is already a command and is still not working.