Hello Tuan,
This error 404 because of you email haven't associated with an account or don't have an valid Service contract. But you could still open a case through email(still need to check the service contract for the tech support). Please view the GTAC user guide from below link for more detail about how to open case by email. https://gtacknowledge.extremenetworks.com/articles/Q_A/Where-is-the-GTAC-User-Guide