Auto move Enemy_script

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

public class AutoMoveEntity : MonoBehaviour
{
    private Vector3 target;
    private Terrain myTerrain;
    private float speed;
    void Start()
    {
        if(speed < 0)
        {
            speed = Random.Range(2.0f, 4.0f);
        }
        myTerrain=Terrain.activeTerrain;
        target=ReturnTarget();

    // Update is called once per frame
    void Update()
    {
        if(transform.position != target)
        {
            transform.position = Vector3.MoveTowards(transform.position, target, speed * Time.deltaTime);
        }
        else
        {
            target = ReturnTarget();
        }
    }

    Vector3 ReturnTarget()
    {
        float x = Random.Range(transform.localScale.x/2, myTerrain.terrainData.size.x - transform.localScale.x/2);
        float z = Random.Range(transform.localScale.z/2, myTerrain.terrainData.size.z - transform.localScale.z/2);

        Vector3 move=new Vector3(x, transform.localScale.y/2, z);
        return move;
    }

    private void OnCollisionEnter(Collision other) 
    {
        target=ReturnTarget();
    }
}