Checkout pane doesn't display custom panes

I'm using the documentation example. For some reason, I cannot get the custom pane to display in the checkout.

What am I missing?

I'm using the latest version of Drupal Commerce and Drupal 9.2.

namespace Drupal\my_checkout_pane\Plugin\Commerce\CheckoutPane;

use Drupal\commerce_checkout\Plugin\Commerce\CheckoutPane\CheckoutPaneBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Provides a custom message pane.
 *
 * @CommerceCheckoutPane(
 *   id = "my_checkout_pane_custom_message",
 *   label = @Translation("Custom message"),
 *   display_label = @Translation("Another display label"),
 *   default_step = "review",
 *   wrapper_element = "fieldset",
 * )
 */
class CustomMessagePane extends CheckoutPaneBase {

  /**
   * {@inheritdoc}
   */
  public function buildPaneForm(array $pane_form, FormStateInterface $form_state, array &$complete_form) {
    $form_display = EntityFormDisplay::collectRenderDisplay($this->order, 'checkout');
    $form_display->buildForm($this->order, $pane_form, $form_state);
    return $pane_form;
  }

  /**
   * {@inheritdoc}
   */
  public function validatePaneForm(array &$pane_form, FormStateInterface $form_state, array &$complete_form) {
    $form_display = EntityFormDisplay::collectRenderDisplay($this->order, 'checkout');
    $form_display->extractFormValues($this->order, $pane_form, $form_state);
    $form_display->validateFormValues($this->order, $pane_form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitPaneForm(array &$pane_form, FormStateInterface $form_state, array &$complete_form) {
    $form_display = EntityFormDisplay::collectRenderDisplay($this->order, 'checkout');
    $form_display->extractFormValues($this->order, $pane_form, $form_state);
  }

}