That is USPS lingo for "there is something we don't like about the address(es)." Go back to Step 1 and Step 2 and make sure you have filled in all required (*) fields with appropriate information.
If you still have issues, try logging out and then logging back in. That will force you to have type in your info again on Step 2 but usually fixes the problem.
If you still have issues after that, please open a help ticket with your User# and the name of the Mailing Profile you're choosing on Step 1. Also include the address for which you're trying to create a label. In rare cases, that means USPS does not offer the Certified Mail service for the address.