Variable target in 'Targetting system' error.

Hi. I've been following the tutorials of BergZergArcade (Look him up on Youtube, he's awesome', but i came across this error:

UnassignedReferenceException: The variable target of 'PlayerAttack' has not been assigned. You probably need to assign the target variable of the PlayerAttack script in the inspector. PlayerAttack.Attack () (at Assets/Scripts/PlayerAttack.cs:33) PlayerAttack.Update () (at Assets/Scripts/PlayerAttack.cs:25)

My script:

using UnityEngine; using System.Collections; using System.Collections.Generic;

public class Targetting : MonoBehaviour { public List targets; public Transform selectedTarget;

 private Transform myTransform;
 
 // Use this for initialization
 void Start () {
     targets = new List();
     selectedTarget = null;
     myTransform = transform;
     
     AddAllEneies();
 }
 
 public void AddAllEneies()
 {
     GameObject[] go = GameObject.FindGameObjectsWithTag("Enemy");
 
     foreach(GameObject enemy in go)
         AddTarget(enemy.transform);
 }
 
 public void AddTarget(Transform enemy)
 {
     targets.Add(enemy);
 }
 
 private void SortTargetByDistance()
 {
     targets.Sort (delegate(Transform t1, Transform t2) { 
             return Vector3.Distance(t1.position, myTransform.position).CompareTo(Vector3.Distance(t2.position, myTransform.position));
             });
 }
 
     
 private void TargetEnemy()
 {
     if(selectedTarget == null)
     {
         SortTargetByDistance();
         selectedTarget = targets[0];
     }
     else    
     {
         int index = targets.IndexOf(selectedTarget);
         
         if(index < targets.Count - 1)
         {
             index++;
         }
         else
         {
             index = 0;    
         }
         DeselectTarget();
         selectedTarget = targets[index];
     }
     SelectTarget();
 }
     
 private void SelectTarget()
 {
     selectedTarget.renderer.material.color = Color.red;
     
     PlayerAttack pa = (PlayerAttack)GetComponent("Playerattack");
     
     pa.target = selectedTarget.gameObject;
 }
 
 private void DeselectTarget()
 {
     selectedTarget.renderer.material.color = Color.green;
     selectedTarget = null;
 }
 
 // Update is called once per frame
 void Update () {
     if(Input.GetKeyDown(KeyCode.Tab))
     {
         TargetEnemy();    
     }
 }

}

So basically, i have a script called 'PlayerAttack' attached to my player. But i have 3 evil cubes, so how do i select more than one to be attached to the PlayerAttack script so i can attack all three. And last but not least, what if i get for example over 100 enemies, do i have to attach them all to the script? I hope some of you can help me out. Thanks alot!