Hey, as many as you know, it’s pretty hard to book a seat reservation in Spain/France, especially online. Well, even when you come across an available seat reservation, it always requires a paper pass. I was having the same problem a few days ago until I came across a reply to one of the questions on here. I was told to go to this site https://www.sncf.com/en/customer-service/contact-us/telephone

Dial  00 33 1 84 94 3635, once they answer input #85 to listen to it in english. After a representative picks up the phone, you tell them what pass you have, what date you are planning on going etc, once they find a reservation for you, they will have you input your credit card details etc and once that is also done, you’ll receive an email regarding your reservation. I just booked Paris → Barcelona for €29.

Let me know if you have any questions.