Have a look here :

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

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

As a rule of thumb : cross-border trains are very expensive so if you'd like to save some money, avoid them.

- Barcelona - Paris : 37€ very popular, some dates are already sold out. If you want to take the direct train book it ASAP.

Alternatives :

- regional trains Barcelona - Cerbère - Perpignan or Montpellier (reservation-free)

- TGV Perpignan or Montpellier - Paris (limited 10€ fare, then 20€)

It takes longer but the route is scenic.

OR

- regional train Barcelona - Cerbère (Mediterranean) or Latour de Carol (Pyrénées)

- from either station night train to Paris (couchette 23-25€)