If you sign in on the Support Panel Desktop but the agent isn't Online maybe you have a configuration issue with the PHP.
Please follow the next steps provided by norton :
The sessions folder referenced by php was not where it was stated, so you to add two lines of code to the config.php file so its loaded on all pages.
session_save_path('/var/www/vhosts/mysite.com/httpdocs/session'); ini_set('session.gc_probability', 1);
Also make the folder 777