Any possibility that there is a purchase / parental control PIN set that is preventing you from purchasing? Even though I am the only one in my home I still have a purchase PIN set so I can't accidentally order something. Otherwise, no idea.
Theres no place to put a pin in it just says its not authorized.
The few times I have heard of this is when the FiOS service is on a business account. If you have business class FiOS Internet, PPV is restricted, but otherwise should work.
Possibly if there was ever an issue with your biling or payment, this could possibly be restricting it also. Like if your bill is passed due. Although I have never heard of this, and I imagine it is not the case.