That may be the problem. Some reservations can only be made 2 months in advance, others 3 or 4 months and some 6 months.
For more information we need to know which trains exactly.
Somewhere on the site is also an extensive list (perhaps too daunting for those not used to trains here in EUR) that gives details -like these-and the usual cost and also IF it really is needed/mandatory or not- for all countries and participating train companies.
In quite a few to make the REServation is not even possible via the site or app, you have to visit a counter in the country concerned.
The train was the TGV from Paris Montparnasse to Lourdes France on July 13.
The train was the TGV from Paris Montparnasse to Lourdes France on July 13.
Reservation in France normally opens 3 months in advance.