How to display an error message if the box is not checked?

The following code for Drupal 8, displays in Drupal Commerce a check box to accept the terms of sales during the order.

The checkbox is mandatory to complete the purchase.

The code works, but I want to display an error message if the box is not ticked.

I add a code at the end, but it is not correct

How to display an error message if the box is not checked ?

<?php

namespace Drupal\commerce_agree_cgv\Plugin\Commerce\CheckoutPane;

use Drupal\Component\Serialization\Json;
use Drupal\Core\Form\FormStateInterface;
use Drupal\commerce_checkout\Plugin\Commerce\CheckoutPane\CheckoutPaneBase;
use Drupal\commerce_checkout\Plugin\Commerce\CheckoutPane\CheckoutPaneInterface;
use Drupal\Core\Link;
use Drupal\Core\Url;

/**
 * Provides the completion message pane.
 *
 * @CommerceCheckoutPane(
 *   id = "agree_cgv",
 *   label = @Translation("Agree CGV"),
 *   default_step = "review",
 * )
 */
class AgreeCGV extends CheckoutPaneBase implements CheckoutPaneInterface {

  /**
   * {@inheritdoc}
   */
  public function buildPaneForm(array $pane_form, FormStateInterface $form_state, array &$complete_form) {
    $store_name = $this->order->getStore()->getName();
    $store_id = $this->order->getStoreId();
    $pane_form['#attached']['library'][] = 'core/drupal.dialog.ajax';
    $attributes = [
      'attributes' => [
        'class' => 'use-ajax',
        'data-dialog-type' => 'modal',
        'data-dialog-options' => Json::encode([
          'width' => auto,
        ]),
      ],
    ];
    $link = Link::fromTextAndUrl(
      $this->t('the general terms and conditions of the store "@store_name"', ['@store_name' => $store_name]),
      Url::fromUri("internal:/store/$store_id/cgv", $attributes)
    )->toString();
    $pane_form['cgv'] = [
      '#type' => 'checkbox',
      '#default_value' => FALSE,
      '#title' => $this->t('I have read and accept @cgv.', ['@cgv' => $link]),
      '#required' => TRUE,
      '#weight' => $this->getWeight(),
    ];
    return $pane_form;
  }

  /**
   * {@inheritdoc}
   */
  public function validatePaneForm(array &$pane_form, FormStateInterface $form_state, array &$complete_form) {
    if (!$values['cgv']) {
      $form_state->setError['You must agree with the @terms before continuing'];
    }
  }

}