Deleting objects at specific position

Hey, I'm trying to make my objects delete themselves after they reach a certain value on an axis, however when I try to attach my transform to the script in the Unity Editor, it doesn't let me. Below is the script that I'm using in case that is the problem.

 using UnityEngine;
 using System.Collections;
 
 public class WaterDestroy : MonoBehaviour 
 {
 
     public Transform deletePos;
     // Use this for initialization
     void Start () 
     {
 
     }
     
     // Update is called once per frame
     void Update () 
     {
         if (transform.position.y <= deletePos.position.y) 
         {
             Destroy (this.gameObject);
         }
     }
 }

Please help, this is for an assessment.