As an update, I was able to reach an agent on the phone. I was trying to purchase "kids fly free" tickets, which are clearly an option on the Frontier website for the day of my flight (several flights that day did not have the option, but two flights did). The phone agent told me that the date I was flying (December 28) was a blackout date for kids fly free. He showed me the "terms and conditions" page on the website that did, indeed, list December 27-28 as blackout dates. Why they would then advertise December 28 as a "kids fly free" option is unknown to me. Regardless, he booked the flight for me (paying for kids) and waived the booking fee; he also gave me the discount den price for the tickets, which saved about $5/person/each way. I don't know if this is the problem that caused the error message, but once I discovered that the kids fly free wasn't an option I became uninterested in fixing the glitch and just wanted to be done.