Script not working, help please

So this is kinda the continuation of this thread: http://answers.unity3d.com/questions/495573/use-of-unassigned-variable-script-from-script-refe.html which I already got help and the error disappeared but the script doesn't works so I've decided to make a new thread for it, this is my script: using UnityEngine; using System.Collections;

 public class EnemyScript : MonoBehaviour {
 
     public float health = 100;
     public float speed = 100;
     
     public GameObject[] gos;
     public GameObject closestPlayer = null;
     public float distance;
     
     void FixedUpdate(){
         transform.LookAt(closestPlayer.transform.position);
         FindClosestEnemy();
     }
     
     GameObject FindClosestEnemy() {
         gos = GameObject.FindGameObjectsWithTag("Player");
         distance = Mathf.Infinity;
         foreach (GameObject go in gos) {
             Vector3 diff = go.transform.position - transform.position;
             float curDistance = diff.sqrMagnitude;
             if (curDistance < distance) {
                 closestPlayer = go;
                 distance = curDistance;
             }
         }
         return closestPlayer;
     }
 }

The script isn't finding the closest Player, so I'm getting this error: UnassignedReferenceException: The variable closestPlayer of 'EnemyScript' has not been assigned. You probably need to assign the closestPlayer variable of the EnemyScript script in the inspector. EnemyScript.FixedUpdate () (at Assets/Scripts/EnemyScript.cs:14)

Help me please, and thanks in advance