You can buy reservations for the TGV/AVE France-Barcelona via Deutsche Bahn Reisezentrum

You can also buy it at SNCF Desks in France.

P.S. but only some seats per train are possible to use with Interrail, and so the reservations can get very soon sold out.

P.P.S. Maybe you have to try some DB Reisezentrum (Ticket Office), because not all people who works there knows how it works to do the reservation. In bigger stations, they should know. 

Okay thanks a lot

Do you know if they still send you the reservations by post or if i can use them only digital or print at home ?

You have to go to the Ticket Office and you get reservations on paper.

do you mean Interrail or DB?

Reservations for this trains between France and Spain are only on paper. 

Okay thanks for letting me know

i wasn’t sure if train connections in spain and france can be booked via deutsche Bahn, but i‘ll try it. and it’s just the reservation for the interrail pass

Via DB you can buy reservations for TGV/ICE/Thalys from Germany to France (except TGV Freiburg-Paris) and TGV/AVE France-Barcelona. Trains within France are not possible to get via DB. 

Not anymore unfortunately. DB used to sell reservations for almost every train in Europe :(

Thanks to everyone 

i was able to book my last two reservations at the DB Reisezentrum in Munich :)