Can't access a variable from a prefab unity script

Hello !

I came across a strange problem today. I've searched for some times but found nothing about my problem. I think i'm just missing something but can't understand what.

Here's the thing : for a basic school project, i'm using the AIthirdperson controller, found in the standard asset. With this prefab (and script), you just have to drop a Transform in the variable "target" and the character will walk to that transform. I wanted to change this transform via another script but when I'm trying to access the "AICharacterControl" script i get an error "this name doesn't exist in the current context"...Why ?

Thanks!

     using UnityEngine;
     using System.Collections;
 using System;
     
     public class waypoint_test : MonoBehaviour {
     
         public GameObject PNJ;
         public AICharacterControl target_variable;
      
     
         // Use this for initialization
         void Start () {
     
     
         }
         
         // Update is called once per frame
         void Update () {
         
         }
     }

The AI script

      using System;
     using UnityEngine;
      using System.Collections;
     
     namespace UnityStandardAssets.Characters.ThirdPerson
     {
         [RequireComponent(typeof (NavMeshAgent))]
         [RequireComponent(typeof (ThirdPersonCharacter))]
         public class AICharacterControl : MonoBehaviour
         {
             public NavMeshAgent agent { get; private set; }             // the navmesh agent required for the path finding
             public ThirdPersonCharacter character { get; private set; } // the character we are controlling
             public Transform target;                                    // target to aim for
     
     
             private void Start()
             {
                 // get the components on the object we need ( should not be null due to require component so no need to check )
                 agent = GetComponentInChildren<NavMeshAgent>();
                 character = GetComponent<ThirdPersonCharacter>();
     
                 agent.updateRotation = false;
                 agent.updatePosition = true;
             }
     
     
             private void Update()
             {
                 if (target != null)
                     agent.SetDestination(target.position);
     
                 if (agent.remainingDistance > agent.stoppingDistance)
                     character.Move(agent.desiredVelocity, false, false);
                 else
                     character.Move(Vector3.zero, false, false);
             }
     
     
             public void SetTarget(Transform target)
             {
                 this.target = target;
             }
         }
     }