UnassignedReferenceException: The variable enemy of EnemiesSpawner has not been assigned unity3d

I am getting this error every time when I play my scene :

UnassignedReferenceException: The variable enemy of EnemiesSpawner has not been assigned. You probably need to assign the enemy variable of the EnemiesSpawner script in the inspector.

 using UnityEngine;
  using System.Collections;
  
  public class EnemiesSpawner : MonoBehaviour {
  public GameObject enemy;
  public Transform [] spawnPoints;
  public float spawnTime = 5f;
  public Vector3 spawnValues;
     
      void Start () {
       InvokeRepeating("Spawn", spawnTime, spawnTime); //Calls the "Spawn" function every 10 seconds.
  
      }
      
  
      void Spawn () {
      
   int spawnPointIndex = Random.Range (0, spawnPoints.Length); for( int spawnCount = spawnPoints.Length - 1 ; spawnCount >= 0 ; --spawnCount )
                   Instantiate(enemy, spawnPoints[spawnPointIndex].position, spawnPoints[spawnPointIndex].rotation);
                    Vector3 spawnPosition = new Vector3 (Random.Range (-spawnValues.x, spawnValues.x), 1, Random.Range (-spawnValues.z, -spawnValues.z) ) ; // <= here
     
 
                   
     
  
      
      }
  }