Simplybook book(). Instance not defined error

I'm relatively new to programming and I have to create a booking system for the company I work at using Simplybook.me

This is the code I've tried

jQuery(document).ready(function() {
  var loginClient = new JSONRpcClient({
    'url': 'https://user-api.simplybook.me' + '/login', //Connects to the API
    'onerror': function(error) {
      instance.error(error);
    }
  });

  client = null,
    $company = "COMPANY";
  $user = "USER"; //Fill in this data. Name of the account: email
  $pass = "PASS"; //Fill in this data. Password

  var token = loginClient.getUserToken($company, $user, $pass);
  this.client = new JSONRpcClient({
    'url': 'https://user-api.simplybook.me' + '/admin/',
    'headers': {
      'X-Company-Login': $company,
      'X-User-Token': token
    },
    'onerror': function(error) {
      instance.error(error);
    }
  });

  // Booking trial (Danger)

  $additionalFieldsActivated = this.client.isPluginActivated('event_field');
  //console.log($additionalFieldsActivated); // FALSE

  //$additionalFields = this.client.getAdditionalFields();
  $additionalFields = {};

  //console.log($additionalFields);
  //console.log($additional);

  var eventId = 170;
  var unitId = 35; // Luis Solana unit (provider) code
  var date = "2023-12-12";
  var time = "11:11:11";
  var clientData = {
    'id': 293,
    'name': "Luis" + " " + "Solana",
    'email': "EMAIL",
    'phone': "+34XXXXXXXXX",
  };
  //$clientData = 293;

  console.log(clientData);

  $booking = new Object();
  $booking = this.client.book(eventId, unitId, date, time, clientData, $additionalFields);
  console.log($booking);
  // Logs

  $servicesList = this.client.getCategoriesList();
  $clientList = this.client.getClientList();
  $unitList = this.client.getUnitList();
  $eventList = this.client.getEventList();
  //console.log($clientList);
});

However, an error pops.

Uncaught ReferenceError: instance is not defined
    onerror file:///C:/Users/lsolana/Desktop/DATA TASAP/Software/TASAP_Bookings_a1/js/query.js:29
    success file:///C:/Users/lsolana/Desktop/DATA TASAP/Software/TASAP_Bookings_a1/js/json-rpc-client.js:100
    jQuery 6
    method file:///C:/Users/lsolana/Desktop/DATA TASAP/Software/TASAP_Bookings_a1/js/json-rpc-client.js:88
    <anonymous> file:///C:/Users/lsolana/Desktop/DATA TASAP/Software/TASAP_Bookings_a1/js/query.js:62
    jQuery 8

I've tried console.log()-ing $booking, and it says it doesn't find the client details, but I believe it is because of the preceding code.

Thanks!

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