NOT possible online, plus that this route=direct is very often heavily oversubscribed. It is very much possible and with much lower cost to use local trains IN ESpana rodalies till Cerbere (=1st stop In the FRancia), then local to Perpignan/Narbonne, then TGV to Paris=save some 20+ € and much more chance on finding seats and bookable online.

We have to tell this little secret about a 100 times every day-as it seems each&every newbee wants only to use this direct TGV.

Thank you so much for the information. I didn't know you say about to travel to Perpignan.   Could you tell if to brussels there is any other route without reservation from London? It is my firts time traveling for Interail I am traveling from Colombia.

Where did you order the reservation Barcelona-Paris? If via Eurail, then it will be sent via the post. That order should be visible in your reservations overview though.

For London - Brussels, there is only the Eurostar, with mandatory reservations (can be booked online).