Error: "Combination of Services not Available"
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.