Renfe (Spanish Rail) can be difficult to reserve. 

Bestway at a International Ticket office, via Phone of Renfe (pick up within 72hrs) or via a Reservation service like Raileurope of ACP Rail

Hi Anna! You cannot reserve these trains online. However, there are a few, slightly less convenient, options:

• at the station, when you're in Spain (seats for your specific train might not be available anymore, but trains on your routes are plenty)
• call Renfe (the Spanish railway company), and collect the reservations at a Spanish train station within 72 hours.
• call Deutsche Bahn. (if you still have plenty of time) They should be able to reserve for you, and they'll send the tickets by post. (a small booking fee applies) Very friendly personell!
• You might be able to try at a train station with a ticket office in France, but I'm not sure if SNCF is able to book trains in Spain.

Edit: yes, ACPRail is also a great possibility, where you can make your reservations online. However, in my opinion they charge way too much. For example, for an Arco/Alvia/Avant, they charge €10, which is normally €6,50. When you have only one reservation that's okay, but with multiple reservations it adds up eventually.