How can Drupal Commerce Stock email admin when stock is 0?

Drupal: 9.5.10 Commerce: 8.x-2.36 Commerce Stock: 8.x-1.1

I'd like to email the admin when the stock is 0.
In one option, I've tried to set a condition in Rules based on the stock field but I can't get the field to display in "Entity has Bundle" Other options?

By looking at the code proposed by the module commerce_stock I believe you'll have to write a new EventSubscriber that subscribe to the event onOrderComplete.

So you create the file: modules/custom/my_custom_commerce_stock/src/EventSubscriber/SendEmailWhenNoStockSubscriber.php

<?php

namespace Drupal\my_custom_commerce_stock\EventSubscriber;

class SendEmailWhenNoStockSubscriber implements EventSubscriberInterface { 

  public static function getSubscribedEvents() { 
    'commerce_order.post_transition' => 'onOrderComplete', 
  }

  public function onOrderComplete(WorkflowTransitionEvent $event) {
    // code to send email.  
  }

}

If you don't know how to send an email with Drupal, this is very well documented:

• https://www.lucius.digital/en/blog/sending-html-mails-drupal-89-programmatically-example-drupal-module-including-twig-template

If you don't want to code, you can try the following module https://www.drupal.org/project/commerce_stock_notifications