Create a post method in Drupal 8: how to receive data?

Sorry for the title :(

Now I write a serialization class:

namespace Drupal\custom_rest\Plugin\rest\Annotation;

use \Drupal\Component\Annotation\Plugin;

/**
 * Defines a REST resource annotation object.
 */
class UserResource extends Plugin {
  /**
   * The resource plugin username.
   *
   * @var string
   */
  public $username;
  
}

And in the annotation:

/**
 * Provides a resource to get view modes by entity and bundle.
 *
 * @RestResource(
 *   id = "custom_rest_resource_user_check",
 *   label = @Translation("Custom User Check"),
 *   serialization_class = "Drupal\custom_rest\Plugin\rest\Annotation\UserResource",
 *   uri_paths = {
 *     "canonical" = "/api/v1/custom/user_check",
 *     "https://www.drupal.org/link-relations/create" = "/api/v1.0/custom/user_check"
 *   }
 * )
 */

In the method:

public function post(UserResource $user) {
    return new ResourceResponse($user->username);
  }

But now I got this error for postman:

Already implement serialization_class.

Here my implementation:

namespace Drupal\custom_rest\Plugin\rest\resource;

use Drupal\custom_rest\Plugin\rest\annotation\TestResource;
use Drupal\rest\Plugin\ResourceBase;
use Drupal\rest\ResourceResponse;


/**
 * Provides a resource to get view modes by entity and bundle.
 *
 * @RestResource(
 *   id = "custom_rest_resource_test",
 *   label = @Translation("Test Rest"),
 *   serialization_class = "Drupal\custom_rest\Plugin\rest\annotation\TestResource",
 *   uri_paths = {
 *     "canonical" = "/api/v1.0/custom/test",
 *     "https://www.drupal.org/link-relations/create" = "/api/v1.0/custom/test"
 *   }
 * )
 */
class RestResourceTest extends ResourceBase {
  /**
   * Responds to POST requests.
   */
  public function post(TestResource $test) {
    return new ResourceResponse($test->title);
  }

}

My Normalize class:

namespace Drupal\custom_rest\Plugin\rest\normalizer;

use Drupal\custom_rest\Plugin\rest\annotation\TestResource;
use Drupal\serialization\Normalizer\NormalizerBase;

class TestDenormalizer extends NormalizerBase   {
  /**
   * The interface or class that this Normalizer supports.
   *
   * @var array
   */
  protected $supportedInterfaceOrClass = ['Drupal\custom_rest\Plugin\rest\annotation\TestResource'];

  /**
   * Denormalizes data back into an object of the given class.
   *
   * @param mixed $data data to restore
   * @param string $class the expected class to instantiate
   * @param string $format format the given data was extracted from
   * @param array $context options available to the denormalizer
   *
   * @return object
   */
  public function denormalize($data, $class, $format = NULL, array $context = array()) {
    $test = new TestResource();

    $test->title = $data['title'];

    return $test;
  }

  /**
   * Normalizes an object into a set of arrays/scalars.
   *
   * @param object $object object to normalize
   * @param string $format format the normalization result will be encoded as
   * @param array $context Context options for the normalizer
   *
   * @return array
   */
  public function normalize($object, $format = NULL, array $context = array()) {
    // TODO: Implement normalize() method.
  }
}

My annotation class:

namespace Drupal\custom_rest\Plugin\rest\annotation;

/**
 * Defines a REST resource annotation object.
 */
class TestResource {
  /**
   * The resource plugin title.
   *
   * @var string
   */
  public $title;

  public function __construct() {

  }

}

My services.yml

services:
  custom_rest.denormalizer.test:
    class: Drupal\custom_rest\Plugin\rest\normalizer\TestDenormalizer
    tags:
      - { name: normalizer }

I have activated:

I tested this functionality in Postman but I get the following error:

@Wim Leers I changed the serialization class but dont work :(

I attached the module.
Thank you.

Thank you so much @dawehner