$autoload['libraries'] = array('session'); error

The error message literally tells you what the problem is.

From my config file:

$config['sess_driver'] = 'database';
$config['sess_cookie_name'] = 'session';
$config['sess_expiration'] = 7200;
$config['sess_save_path'] = 'sessions';
$config['sess_match_ip'] = TRUE;
$config['sess_time_to_update'] = 300;
$config['sess_regenerate_destroy'] = FALSE;

I modified it as per the instructions in the user guide.
I also created the database table as specified.

Thanks.