TGV fron Paris to Barcelona- Help!

The reason that you can't get past 12 December is that new timetables will go into effect throughout most of Europe on that date. While the timetables are not likely to change by more that a few minutes (if they change at all), you won't be able to book tickets until the new timetables are loaded on each country's national rail site. That usually happens in early November. Beginning in the middle of October, start checking once a week until the updated timetables appear.

Tim is correct. However, services on that route will see a milestone with the new timetable, with the first experimental services beginning on the new high-speed line from Perpignan to Figueres.