Why don't my enemies walk toward me?

the enemies use the animation but they don't walk, help

this is the script:

 using UnityEngine;
 using System.Collections;
 
 namespace CompleteProject
 {
     public class EnemyMovement : MonoBehaviour
     {
         Transform player;               // Reference to the player's position.
         PlayerHealth playerHealth;      // Reference to the player's health.
         EnemyHealth enemyHealth;        // Reference to this enemy's health.
         NavMeshAgent nav;               // Reference to the nav mesh agent.

         void Awake ()
         {
             // Set up the references.
             player = GameObject.FindGameObjectWithTag ("Player").transform;
             playerHealth = player.GetComponent <PlayerHealth> ();
             enemyHealth = GetComponent <EnemyHealth> ();
             nav = GetComponent <NavMeshAgent> ();
         }
 
         void Update ()
         {
             // If the enemy and the player have health left...
             if(enemyHealth.currentHealth > 0 && playerHealth.currentHealth > 0)
             {
                 // ... set the destination of the nav mesh agent to the player.
                 nav.SetDestination (player.position);
             }
             // Otherwise...
             else
             {
                 // ... disable the nav mesh agent.
                 nav.enabled = false;
             }
         }
     }
 }