You have to check if the account is link to the support id that the customer is using in logging a support case
Verify if the contact is added as the technical contact of the company. If not, customer may go to the site below to manage technical contact. Once the request of adding or replacing technical contact, the support ID will now be associated to the account
Please note that the product associated with the suport id should have valid support as well. This is one factor why the support id is invalid
A customer of mine also experienced the same issue. Support ID is valid and has valid support. What we did was, we used the contact ID associated to his email address. He manage to create a case.
If you want to try creating technical case using your contact ID, kindly pm me your email address that is registered with Symantec and the company name.