Your script is fine and it should display messages in console but OnTriggerEnter2D method will only be called when an object with rigidbody or collider enters another object collider range (also assuming you have checked OnTrigger option on collider). I suggest you read about how to use OnTriggerEnter2D method. For now, you can change your script to something like
public class HealthCollectible : MonoBehaviour
{
void Start()
{
Debug.Log("Start method");
health();
}
void health()
{
Debug.Log("health method was called");
}
void OnTriggerEnter2D(Collider2D other)
{
Debug.Log("Object that entered the trigger : " + other);
}
}