use Symfony\Component\Form\Extension\Core\Type\CheckboxType;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;

$builder->addEventListener(FormEvents::PRE_SET_DATA, function (FormEvent $event) {
                $Customer = $event->getData();
                if ($Customer instanceof Customer && !$Customer->getId()) {
                    $form = $event->getForm();
    
                    $form->add('user_policy_check', CheckboxType::class, [
                            'required' => true,
                            'label' => null,
                            'mapped' => false,
                            'constraints' => [
                                new Assert\NotBlank(),
                            ],
                        ]);
                }
            }
            );

<div class="ec-RegisterRole__actions">
                
                <div class="ec-off4Grid">
                        
                    <div class="ec-off4Grid__cell">
                       
<div class="ec-checkbox{{ has_errors(form.user_policy_check) ? ' error' }}">
                                    <label>
                                        {{ form_widget(form.user_policy_check) }}
                                        {{ '<a class="ec-link" href="%url%" target="_blank">利用規約</a>'|trans({ '%url%': url('help_agreement') })|raw }}
                                        {{ '並びに<a class="ec-link" href="%url%" target="_blank">プライバシーポリシー</a>に同意してお進みください'|trans({ '%url%': url('help_privacy') })|raw }}
                                    </label>
                                    {{ form_errors(form.user_policy_check) }}
                                </div>
                         
                        <button type="submit" class="ec-blockBtn--action">{{ '次へ'|trans }}</button>
                        <a class="ec-blockBtn--cancel" href="{{ url('cart') }}">{{ '戻る'|trans }}</a>
                    </div>
                </div>
                
                
            </div>

An exception has been thrown during the rendering of a template (\"\").","ec-root/app/template/default/Shopping/nonmember.twig",163

$builder->addEventListener(FormEvents::PRE_SET_DATA, function (FormEvent $event) {
               //$Customer = $event->getData();
                //if ($Customer instanceof Customer && !$Customer->getId()) {
                    $form = $event->getForm();
    
                    $form->add('user_policy_check', CheckboxType::class, [
                            'required' => true,
                            'label' => null,
                            'mapped' => false,
                            'constraints' => [
                                new Assert\NotBlank(),
                            ],
                        ]);
                //}
            }
 );