get_users – Sort by a different meta_value than search criteria

Should be 'meta_key' => 'last_name' instead of 'meta_key' => 'user_lastname'

$args = array(
    'meta_query' => array(
        array(
            'key' => 'pw_user_status',
            'value' => 'approved' ,
        ),
    ),
    'orderby' => 'meta_value',
    'meta_key' => 'last_name',
    'order' => 'ASC'
);
$users = get_users( $args );