Travel dates would be helpful.

Sorry , finding complicated… July 24 leaving Paris to Lourdes France, then leaving Lourdes on July 25 to Santiago de Compostela, Spain.

The easy bit first :

• TGV Paris-Montparnasse - Lourdes 06:35 - 11:21 10€ OR
• TGV Paris-Montparnasse - Lourdes 10:08 - 14:58 10€

Book on interrail.eu from 90 days in advance (2€ booking fee) or on https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish (4€ booking fee per order)

Spain will be more complicated though…

• TER Lourdes - Bayonne 06:33 - 08:21 reservation-free
• TER Bayonne - Hendaye 08:29 - 09:07 reservation-free
• Euskotren Hendaye - San Sebastian 09:33 - 10:10 2,70€ buy at the station (pass not valid). This train runs every 30 min.
• 15 min walk between the 2 stations

You surely could buy the reservations at San Sebastian RENFE train station (after the walking), see advice.

• IC San Sebastian - Vitoria/Gasteiz 12:13 - 13:58
• ARC Vitoria/Gasteiz - Santiago de Compostela 14:19 - 22:27

San Sebastian would be a better place to spend the night if Lourdes isn’t mandatory.

Reservations for Spain, see advice here :

https://www.seat61.com/interrail-and-eurail-reservations.htm#Spain