A party of fifteen tried to book online and the form refused. Why?

Online bookings are capped at twelve people, and bigger parties are asked to call you. That is deliberate: a table of fifteen is a conversation about layout and timing, not a form. Take it yourself with the Book form at the bottom of Orders → Reservations & waitlist, which accepts up to sixty people, and give it tables when they arrive. Guests can book up to 120 days ahead either way. The Tables, tabs and the kitchen screen tab in the docs lists the limits.