Joomla 3.9.14

I have an external script allowing users to register as a user on my Joomla site. It all appears to be working apart from error displaying. My code is below;

**register.php**

    <?php 
    // database connection
    include('conn.php');
    
    require_once JPATH_BASE .'/components/com_users/models/registration.php';
    
    $model = new UsersModelRegistration();
    
    jimport('joomla.mail.helper');
    jimport('joomla.user.helper');
    
    $language = JFactory::getLanguage();
    $language->load('com_users', JPATH_SITE);
    
    // hard coded for now, in live site these will be $_POST values
    $username = 'adam';
    $name = 'adam24';
    $email = '[email protected]';
    $password = 'password';
    
    $data = array( 
        'username' => $username,
        'name' => $name,
        'email1' => $email,
        'password1' => $password, // First password field
        'password2' => $password, // Confirm password field
        'groups'    =>array('10'),
        'block' => 0 );
    
    echo $model->register($data);
    ?>

When I visit this page, the user is created in my Joomla site with the correct details. When I visit the page for a second time I get an error about the username already being is use, this is expected. However when I dump the error it is displayed as;

    $err = $model->get('_errors');

    // result of var_dump($err)
    array (
      0 => 'Joomla\\CMS\\Form\\Form::getInstance() could not load file',
      1 => 'Registration failed: JLIB_DATABASE_ERROR_USERNAME_INUSE',
    ) 

The `JLIB_DATABASE_ERROR_USERNAME_INUSE` constant isn't loading - why would this be? 

It's defined at `administrator\language\en-GB\en-GB.lib_joomla.ini`