Are you testing your ticketing system in WHMCS? Generally this issue comes up like this : I have just set up an email in WHMCS and I am not able to see the message come through as a ticket request. WHMCS provides an email import log where all the errors are displayed.
Resolving The Ticket ID Not Found Error:
If you have any issues with the ticketing system, you must go to the Utilities >> Logs >> Ticket Mail Import Log in order to view any of the specific errors.
Here you will be able to view the error : ‘Ticket ID Not Found’
If this error occurs, it means that you have sent an email message through your admin email account. Admin email accounts are only expected to reply to the tickets. You must try sending an email through a different email address as might be done by one of your customers and then your message should be displayed as normal.