error:CS0246

这是什么问题啊！
error CS0246: The type or namespace name 'NavMeshAgent' could not be found (are you missing a using directive or an assembly reference?)

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

[RequireComponent(typeof (NavMeshAgent))]
public class Enemy : MonoBehaviour{
NavMeshAgent pathfinder;
Transform target;

void Start(){
    pathfinder = GetComponent<NavMeshAgent>();
    target = GameObject.FindGameObjectWithTag("Player").transform;
}

void Update(){

    pathfinder.SetDestination (target.position);
    
}

}

缺少命名空间引用，加上using UnityEngine.AI