It just says it's an error, and that the date is outside of the timetable period. No further information AT ALL. Instead, I have to manually google to find out why I cannot book a train journey. Turns out the train timetables are changing. Which they seem to do annually.

So literally I cannot book any train tickets for December, which you would think are perhaps maybe possibly the busiest holiday and travel times in the whole year. Maybe folks would want to book those in advance. Just bringing up a small detail they might have missed!