Usually you can make a res for a MNSSHP night, but if it is 7:00 or later in past years it would say you need a party ticket. Since tickets just went on sale today it could be the dining res system needs to be updated with the info so it might not be accepting reservations.
Or it might be a night where reservations are already fully booked (e.g BOG) or the restaurant might not be taking reservations yet (e.g. Cali Grill) or it might not be open (Cali Grill again).
That's my guess (note, we're now on the 3rd new system in ~4 years... you'd think they'd focus on nailing down the quirks instead of making new ones...)
(Also, I never/rarely pass up a chance to bash the IT/Web Development side of Disney )
For the OP:
I'd try calling and see what they say. It could be an issue with the back-end system that would affect both phone and web, or an issue with the front-end web app (which would only affect the web.) (Of course, that's assuming that there isn't an issue with both sides... see the bashing Disney IT/Web Dev comment above )
It's also possible that the restaurant is not open after MNSSHP begins. Many rides and restaurants are closed after 7 pm those nights. They suggest that you eat before arriving to the party.