DB can definitely book reservations from France to Spain and within Spain. Maybe they're very busy now.

An alternative is to contact this DB travel agent. They reply to their e-mail, usually within a business day. Clearly state in your mail which trains on which date(s) you want to book. If you pass through Germany, you can easily collect your reservations from a DB ticket machine.

Thank you so much. Yes, I found an email from Deutsche Bahn already and if they don’t answer I will try the contact you wrote. 

Hope everything works!

Hi ! We have the same problem… We decided to buy interrail pass and then we can’t reserve our seats… We come from Annecy (France) and we want to travel in Madrid.

@Sandy

If you want to make reservations within France and from France to Spain, that can be done through SNCF.

 No need of Pass Cover Number. 

https://www.sncf.com/en/customer-service/contact-us/telephone

 Press #85 for English (no need if you speak French), no booking fees, reservations are sent via e-mail.

If you ive in France, you can also just go to the SNCF ticket office.

As I am now in DE: due to a derailment and a very tragic accident by train (4 people killed) yesterday and the new 9€ ticket DB is simply completely overburdened-so much that its usually quite good booking engine and IT is falling apart or cannot cope anymore with the huge demand. Even the sale of normal tickets in stations is now often not really possible anymore as the system will not respond. So do not pester them with things completely out of their own control.

Hello ! 

We were able to make reservation for our train "Valence (city in France) -Barcelona" but not for "barcelona-Madrid" 

We can't make reservation in France for spanish trains with the "pass interrail" We have to do it at spanish train station ! 

Do you know where make reservation for spanish train before the D-Day ?

Thank you ! 

Please check this post for the known possibilities to book Spanish domestic trains.