TrackableBehaviour / ObserverBehaviour could not be found

Vuforia can track my target image, but I'd like to add a component (new C# script) to the Image target object, such that it waits 3 seconds upon finding the image, and then randomly places one of the 3d (child) objects

Unfortunately, the console shows the error TrackableBehaviour / ObserverBehaviour could not be found.  How can this be done in Unity6?

Delay.cs

using UnityEngine;
using Vuforia;

public class Delay : MonoBehaviour, ITrackableEventHandler
{
 private TrackableBehaviour mTrackableBehaviour;
 public GameObject childObject;

 void Start()
 {
 mTrackableBehaviour = GetComponent<TrackableBehaviour>();
 if (mTrackableBehaviour)
 {
 mTrackableBehaviour.OnTargetStatusChanged += OnTrackableStateChanged;
 }
 }

 public void OnTrackableStateChanged(StatusChangeResult statusChangeResult)
 {
 if (statusChangeResult.NewStatus == Status.TRACKED)
 {
 StartCoroutine(PlaceObjectAfterDelay());
 }
 }

 IEnumerator PlaceObjectAfterDelay()
 {
 yield return new WaitForSeconds(3);
 Vector3 randomPosition = new Vector3(Random.Range(-0.5f, 0.5f), 0, Random.Range(-0.5f, 0.5f));
 childObject.transform.localPosition = randomPosition;
 }
}