SNCF - Train Tickets from Paris to St Jean Pied de Port - What does NOT BOOKABLE mean?

Too far in advance. The new train schedules in Europe come out twice a year: the second Saturday in June and the second Saturday in December. SNCF will sell the tickets after those dates.

As Kenko said about it being too soon. The first week of July just went on sale yesterday so tickets are being sold through that week. Most of the schedule is posted, but more trains could be added. I need to buy tickets for the second week of July and expect to get a notice from SNCF next week.

I don't know anything about schedules posting twice a year. Sometimes we get notices that tickets are on sale for a certain season, but right now it appears to be week-by-week.

I just went on the SNCF site for a different search, and found out that they post schedules twice a year, the early part of December and the early part of June. I am guessing it might be too soon for August. Then again, maybe I am misunderstanding what I read.
Good luck!

I got an email today from SNCF that summer travel tickets will go on sale on Wednesday, March 8. Bon voyage.

Are SNCF tickets refundable? I looked on the fine print - but couldn't tell if they meant fully refundable if cancelled 6 days before travel. Thanks.

Depends on the type of train. Conditions are quite clear when you book.
TGV Inoui (= standard TGV) tickets are fully refundable up to 6 days before departure, with a set fee thereafter.
Ouigo (= low-cost TGV) are not refundable, changes for a fee.
Regional trains vary between regions, sometimes there's a 10% fee and sometimes they are fully refundable.

I saw something on the SNCF website that said that tickets for summer (July and August) will be available March 8, so I’m planning to look then for late July.

I’m still waiting for the full schedule for early May so hoping they will be available March 8.