Enemy wont deal damage

The player deals damage perfectly to an eanmy but the eanmy dont deal damage just says NullReferenceExeption: objact referance not set to an instance of an objact enemy script`using UnityEngine; using System.Collections;

 public class MobAI : MonoBehaviour 
 {
     
     public Transform Player;
 
     public int Speed;
     public int LongRange;
     public int range;
     public int damage;
 
     public AnimationClip run;
     public AnimationClip idle;
     public AnimationClip attack;
 
     public int health;
     public AnimationClip death;
     public GameObject Self;
 
     public GameObject Target;
     
     
     void Start () 
     {
         Target = GameObject.FindWithTag ("Player");
     }
     
     void Update () 
     { 
         if (!isDead ()) 
         {
 
             if (!animation.IsPlaying (attack.name)) 
             {
                 transform.LookAt (Player);
         
                 if (Vector3.Distance (transform.position, Player.position) > range) 
                 {
                     transform.position += transform.forward * Speed * Time.deltaTime;
                     animation.Play (run.name);
                 } 
                 else 
                 {
                     animation.Play (attack.name);
                     Target.GetComponent<PlayerHealth>().getHit(damage);
                 }
             }
 
         }
         else 
         {
             Death ();
         }
     }
     public void getHit (int damage)
     {
         health = health - damage;
     }
     bool isDead ()
     {
         if (health <= 0) 
         {
             return true;
         } 
         else 
         {
             return false;
         }
     }
     void Death ()
     {
         animation.Play(death.name);
         if(animation[death.name].time>animation[death.name].length*0.9)
         {
             Destroy(Self);
         }
     }
 
 }

health code:

 using UnityEngine;
 using System.Collections;
 
 public class PlayerHealth : MonoBehaviour 
 {
     public AnimationClip death;
     public float health;
     public float maxHealth;
     public float healthBarLength;
 
     public GameObject Self;
 
     void start ()
     {
         healthBarLength = Screen.width / 4;
     }
     void update ()
     {
         healthBarLength = (Screen.width / 4)*(health / (float)maxHealth); 
         if (health > 0) 
         {
             ClickToMove.Death = false;
             Damage.Death = false;
         }
         else 
         {
             ClickToMove.Death = true;
             Damage.Death = true;
             animation.Play(death.name);
             if(animation[death.name].time>animation[death.name].length*0.9)
             {
                 Destroy(Self);
             }
         }
     }
     void OnGUI() {
         GUI.backgroundColor = Color.red;
         GUI.Button(new Rect(10, 10, Screen.width / 4 / (maxHealth / health), 20), health + "/" + maxHealth); 
     }
     
     public void getHit (int damage)
     {
         health = health - damage;
     }
 }