Google Admin SDK PHP не может создать пользователя. Не авторизован

У меня возникла проблема с внедрением Google Admin SDK в PHP. Всегда выдает ошибку

Fatal error: Uncaught exception 'Google_Service_Exception' with message 'Error calling POST 
https://www.googleapis.com/admin/directory/v1/users: (403) Not Authorized to access this
 resource/api' in ..\google-api-php-client\src\Google\Http\REST.php on line 79

Вот мой код:

<?php

session_start();
/* * **********************************************
  Make an API request authenticated with a service
  account.
 * ********************************************** */
set_include_path("google-api-php-client/src" . PATH_SEPARATOR . get_include_path());
require_once 'Google/Client.php';
require_once 'Google/Service/Directory.php';

/* * **********************************************
  ATTENTION: Fill in these values! You can get
  them by creating a new Service Account in the
  API console. Be sure to store the key file
  somewhere you can get to it - though in real
  operations you'd want to make sure it wasn't
  accessible from the webserver!
  The name is the email address value provided
  as part of the service account (not your
  address!)
  Make sure the Books API is enabled on this
  account as well, or the call will fail.
 * ********************************************** */
$client_id = 'my_client_id'; //Client ID
$service_account_name = 'email'; //Email Address 
$key_file_location = 'key.p12'; //key.p12

if ($client_id == '<YOUR_CLIENT_ID>' || !strlen($service_account_name) || !strlen($key_file_location)) {
    echo missingServiceAccountDetailsWarning();
}

$client = new Google_Client();
$client->setApplicationName("appname");
$service = new Google_Service_Directory($client);

/* * **********************************************
  If we have an access token, we can carry on.
  Otherwise, we'll get one with the help of an
  assertion credential. In other examples the list
  of scopes was managed by the Client, but here
  we have to list them manually. We also supply
  the service account
 * ********************************************** */
if (isset($_SESSION['service_token'])) {
    $client->setAccessToken($_SESSION['service_token']);
}
$key = file_get_contents($key_file_location);
$cred = new Google_Auth_AssertionCredentials(
        $service_account_name, array('https://www.googleapis.com/auth/admin.directory.user'), $key
);
$client->setAssertionCredentials($cred);
if ($client->getAuth()->isAccessTokenExpired()) {
    $client->getAuth()->refreshTokenWithAssertion($cred);
}
$_SESSION['service_token'] = $client->getAccessToken();


/* * **********************************************
  We're just going to make the same call as in the
  simple query as an example.
 * ********************************************** */    

$familyName = filter_input(INPUT_GET, 'familyname');
$givenName = filter_input(INPUT_GET, 'givenname');
$password = filter_input(INPUT_GET, 'password');
$primaryEmail = filter_input(INPUT_GET, 'email');

$user = new Google_Service_Directory_User();
$name = new Google_Service_Directory_UserName();

$name->setFamilyName($familyName);
$name->setGivenName($givenName);

$user->setName($name);
$user->setHashFunction("MD5");
$user->setPrimaryEmail($primaryEmail);
$user->setPassword(hash("md5", $password));

$result = $service->users->insert($user);

Документации нет, и я очень смущен этой ошибкой.