Unexpected token < in JSON at position 0 error. How to solve?

Question

PHP

When you click on the order check without registering, the fields should be filled in, it doesn’t exit, and after clicking on the continue button, such an error pops up.
guest.php code

<?php
class ControllerCheckoutGuest extends Controller {
  public function index() {
    $this->load->language('checkout/checkout');

    $data['text_select'] = $this->language->get('text_select');
    $data['text_none'] = $this->language->get('text_none');
    $data['text_your_details'] = $this->language->get('text_your_details');
    $data['text_your_account'] = $this->language->get('text_your_account');
    $data['text_your_address'] = $this->language->get('text_your_address');
    $data['text_loading'] = $this->language->get('text_loading');

    $data['entry_firstname'] = $this->language->get('entry_firstname');
    $data['entry_lastname'] = $this->language->get('entry_lastname');
    $data['entry_email'] = $this->language->get('entry_email');
    $data['entry_telephone'] = $this->language->get('entry_telephone');
    $data['entry_fax'] = $this->language->get('entry_fax');
    $data['entry_company'] = $this->language->get('entry_company');
    $data['entry_customer_group'] = $this->language->get('entry_customer_group');
    $data['entry_address_1'] = $this->language->get('entry_address_1');
    $data['entry_address_2'] = $this->language->get('entry_address_2');
    $data['entry_postcode'] = $this->language->get('entry_postcode');
    $data['entry_city'] = $this->language->get('entry_city');
    $data['entry_country'] = $this->language->get('entry_country');
    $data['entry_zone'] = $this->language->get('entry_zone');
    $data['entry_shipping'] = $this->language->get('entry_shipping');

    $data['button_continue'] = $this->language->get('button_continue');
    $data['button_upload'] = $this->language->get('button_upload');

    $data['customer_groups'] = array();

    if (is_array($this->config->get('config_customer_group_display'))) {
      $this->load->model('account/customer_group');

      $customer_groups = $this->model_account_customer_group->getCustomerGroups();

      foreach ($customer_groups as $customer_group) {
        if (in_array($customer_group['customer_group_id'], $this->config->get('config_customer_group_display'))) {
          $data['customer_groups'][] = $customer_group;
        }
      }
    }

    
    if (isset($this->request->post['customer_group_id'])) {
      $data['customer_group_id'] = $this->request->post['customer_group_id'];
    } elseif(isset($this->session->data['guest']['customer_group_id'])) {

            
      $data['customer_group_id'] = $this->session->data['guest']['customer_group_id'];
    } else {
      $data['customer_group_id'] = $this->config->get('config_customer_group_id');
    }
//lightshop start
    $data['button_back'] = $this->language->get('button_back');
    $data['entry_telephone'] = $this->language->get('entry_telephone');
    $data['entry_email'] = $this->language->get('entry_email');
    $data['checkout_st3_sa'] = $this->config->get('theme_lightshop_checkout_st3_sa');
    
    $this->load->model('catalog/lightshop');
    $activeFields = $this->model_catalog_lightshop->getFields($data['customer_group_id']);
    $allFields = $this->model_catalog_lightshop->getAllFields();

    $data['allCustomFields'] = $allFields;
    foreach($allFields as $field){
      $data['entry_'.$field.'_required'] = 0;
      $data['entry_'.$field.'_show'] = 0;
    
      if (isset($this->request->post[$field])) {
        $data[$field] = $this->request->post[$field];
      } elseif (!empty($customer_info)) {
        $data[$field] = $customer_info[$field];
      } else {
        $data[$field] = '';
      }

      if (isset($this->error[$field])) {
        $data['error_'.$field] = $this->error[$field];
      } else {
        $data['error_'.$field] = '';
      }

    }

    foreach($activeFields as $field){
      if($field['description']){
        $data['entry_'.$field['name']] = $field['description'];
      }
      $data['entry_'.$field['name'].'_required'] = $field['required'];
      $data['entry_'.$field['name'].'_show'] = $field['is_show'];
    }
//lightshop end
    if (isset($this->session->data['guest']['firstname'])) {
      $data['firstname'] = $this->session->data['guest']['firstname'];
    } else {
      $data['firstname'] = '';
    }

    if (isset($this->session->data['guest']['lastname'])) {
      $data['lastname'] = $this->session->data['guest']['lastname'];
    } else {
      $data['lastname'] = '';
    }

    if (isset($this->session->data['guest']['email'])) {
      $data['email'] = $this->session->data['guest']['email'];
    } else {
      $data['email'] = '';
    }

    if (isset($this->session->data['guest']['telephone'])) {
      $data['telephone'] = $this->session->data['guest']['telephone'];
    } else {
      $data['telephone'] = '';
    }

    if (isset($this->session->data['guest']['fax'])) {
      $data['fax'] = $this->session->data['guest']['fax'];
    } else {
      $data['fax'] = '';
    }

    if (isset($this->session->data['payment_address']['company'])) {
      $data['company'] = $this->session->data['payment_address']['company'];
    } else {
      $data['company'] = '';
    }

    if (isset($this->session->data['payment_address']['address_1'])) {
      $data['address_1'] = $this->session->data['payment_address']['address_1'];
    } else {
      $data['address_1'] = '';
    }

    if (isset($this->session->data['payment_address']['address_2'])) {
      $data['address_2'] = $this->session->data['payment_address']['address_2'];
    } else {
      $data['address_2'] = '';
    }

    if (isset($this->session->data['payment_address']['postcode'])) {
      $data['postcode'] = $this->session->data['payment_address']['postcode'];
    } elseif (isset($this->session->data['shipping_address']['postcode'])) {
      $data['postcode'] = $this->session->data['shipping_address']['postcode'];
    } else {
      $data['postcode'] = '';
    }

    if (isset($this->session->data['payment_address']['city'])) {
      $data['city'] = $this->session->data['payment_address']['city'];
    } else {
      $data['city'] = '';
    }

    if (isset($this->session->data['payment_address']['country_id'])) {
      $data['country_id'] = $this->session->data['payment_address']['country_id'];
    } elseif (isset($this->session->data['shipping_address']['country_id'])) {
      $data['country_id'] = $this->session->data['shipping_address']['country_id'];
    } else {
      $data['country_id'] = $this->config->get('config_country_id');
    }

    if (isset($this->session->data['payment_address']['zone_id'])) {
      $data['zone_id'] = $this->session->data['payment_address']['zone_id'];
    } elseif (isset($this->session->data['shipping_address']['zone_id'])) {
      $data['zone_id'] = $this->session->data['shipping_address']['zone_id'];
    } else {
      $data['zone_id'] = '';
    }

    $this->load->model('localisation/country');

    $data['countries'] = $this->model_localisation_country->getCountries();

    // Custom Fields
    $this->load->model('account/custom_field');

    $data['custom_fields'] = $this->model_account_custom_field->getCustomFields();

    if (isset($this->session->data['guest']['custom_field'])) {
      if (isset($this->session->data['guest']['custom_field'])) {
        $guest_custom_field = $this->session->data['guest']['custom_field'];
      } else {
        $guest_custom_field = array();
      }

      if (isset($this->session->data['payment_address']['custom_field'])) {
        $address_custom_field = $this->session->data['payment_address']['custom_field'];
      } else {
        $address_custom_field = array();
      }

      $data['guest_custom_field'] = $guest_custom_field + $address_custom_field;
    } else {
      $data['guest_custom_field'] = array();
    }

    $data['shipping_required'] = $this->cart->hasShipping();

    if (isset($this->session->data['guest']['shipping_address'])) {
      $data['shipping_address'] = $this->session->data['guest']['shipping_address'];
    } else {
      $data['shipping_address'] = true;
    }

    // Captcha
    if ($this->config->get($this->config->get('config_captcha') . '_status') && in_array('guest', (array)$this->config->get('config_captcha_page'))) {
      $data['captcha'] = $this->load->controller('extension/captcha/' . $this->config->get('config_captcha'));
    } else {
      $data['captcha'] = '';
    }

    $this->response->setOutput($this->load->view('checkout/guest', $data));
  }

  public function save() {
    $this->load->language('checkout/checkout');

    $json = array();

    // Validate if customer is logged in.
    if ($this->customer->isLogged()) {
      $json['redirect'] = $this->url->link('checkout/checkout', '', true);
    }

    // Validate cart has products and has stock.
    if ((!$this->cart->hasProducts() && empty($this->session->data['vouchers'])) || (!$this->cart->hasStock() && !$this->config->get('config_stock_checkout'))) {
      $json['redirect'] = $this->url->link('checkout/cart');
    }