How do I make my AI move and follow me once I am close enough.

I have recently watched two YouTube tutorials on AI and as I am a beginner with C# I do not understand where it has gone wrong. Can anyone fix the scripting errors it might be due to the conflicts with the two tutorials and how I packed both into one script and did some of my own programming. Thanks. Here is the code:

   using System.Collections;
     using System.Collections.Generic;
     using UnityEngine;
     using UnityEngine.AI;
     public class EnemyController : MonoBehaviour {
         public float fpsTargetDistance;
         public float enemyLookDistance;
         public float attackDistance;
         public float enemyMovementSpeed;
         public float damping;
         public Transform fpsTarget;
         Rigidbody theRigidbody;
         Renderer myRender;
         public float AIWallDistance;
         public Transform AIWall;
         public Transform DeathZone;
         public LayerMask whatIsWall;
         public float MaxDistanceFromWall;
         public Vector3 moveDir;
         void Start(){
             myRender = GetComponent<Renderer> ();
             theRigidbody = GetComponent<Rigidbody> ();
             moveDir = ChooseDirection ();
             transform.rotation = Quaternion.LookRotation (moveDir);
         }
         Vector3 ChooseDirection()
         {
             System.Random ran = new System.Random ();
             int i = ran.Next (0, 3);
             Vector3 temp = new Vector3 ();
             if(i==0){
             
                 temp = transform.forward;
             }
             else if(i==1){
     
                 temp = -transform.forward;
             }
             else if(i==2){
     
                 temp = transform.right;
             }
             else if(i==3){
     
                 temp = -transform.right;
             }
             return temp;
         
         }
         void Update()
         {
             theRigidbody.velocity = moveDir * enemyMovementSpeed;
             if(Physics.Raycast(transform.position, transform.forward,MaxDistanceFromWall))
             {
                 moveDir = ChooseDirection ();
                 transform.rotation = Quaternion.LookRotation (moveDir);
             }
         
         
         
         }
         void FixedUpdate(){
     
             theRigidbody.AddForce (transform.forward * enemyMovementSpeed);
             fpsTargetDistance = Vector3.Distance (fpsTarget.position, transform.position);
         
     
     
             if (fpsTargetDistance < enemyLookDistance) {
                 myRender.material.color = Color.yellow;
                 LookAtPlayer ();
                 theRigidbody.AddForce (transform.forward * enemyMovementSpeed);
             }
             if (fpsTargetDistance < attackDistance) {
                 attack ();
             }
             
         
         }
     
         void LookAtPlayer(){
             Quaternion rotation = Quaternion.LookRotation(fpsTarget.position-transform.position);
             transform.rotation = Quaternion.Slerp (transform.rotation,rotation, Time.deltaTime*damping);
         }
         void attack(){
             theRigidbody.AddForce (transform.forward * enemyMovementSpeed);
         }
     
     
     
     }