ValidationErrorBlock.php

<?php

namespace Drupal\settings_tray_test\Plugin\Block;

use Drupal\Core\Block\Attribute\Block;
use Drupal\Core\Block\BlockBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * Provides a 'Block with validation error' test block.
 */
class ValidationErrorBlock extends BlockBase {
  
  /**
   * {@inheritdoc}
   */
  public function build() {
    return [
      '#markup' => '<span>If I had more time this would be very witty :(.</span>',
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
    parent::validateConfigurationForm($form, $form_state);
    $form_state->setError($form['label'], 'Sorry system error. Save again.');
  }

}