The connection to the Spanish reservations system can be tempramental at the best of times.

You don’t have to buy the reservation from Eurail. There are some suggestions here for other places to try. Unfortunately, the Spanish train operator RENFE makes it very difficult.

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

• To make Spanish passholder reservations, here are the options:

  Option 1, try booking online at the official Interrail/Eurail reservations service.  Until July 2023 it was impossible to book passholder reservations for Spain online anywhere, but the official Interrail/Eurail reservations service started offering them in July 2023, having finally persuaded Renfe to let them do it.  Don't expect too much:  Spanish trains often only open for sale close to departure date, meaning anywhere from 10 to 90 days, and they also have a habit of leaving full.  This system seems to work for some routes/trains, but not necessarily all.  See whether this option works for you for your trains and routes, if it doesn't, try options 2 or 3.

  Option 2, call Deutsche Bahn on +49 30 2970 (German line) or +49 30 311682904 (English-speaking line).  You should first look up the train(s) you want using int.bahn.de and specify the train number(s).  Reservations are issued as old-school hard copy tickets, but these can be sent to any address worldwide by normal post for €5.90 per order or collected from a DB ticket machine in Germany, if you happen to be there.

  Option 3, email andy@railtravelcentre.com, the same rail expert that runs www.discoverbyrail.com.  He can arrange passholder reservations in Spain.  Note that reservations cannot be e-ticketed, they are hard-copy and will only be sent to UK addresses or can be collected from ticket machines in Germany (as the reservations are sourced via the DB system).  His normal handling fee is £17.50, but he'll reduce this to £15 if you quote 'seat61' in your email.  One handling fee is charged 'per booking', where a booking can be multiple passengers reserved on multiple trains.  Remember to tell him if your pass is 1st or 2nd class.

  Option 3, call Renfe telesales on 34 91 232 03 20 and reserve, but (a) it must be more than 24h before departure and (b) you have to collect and pay for the reservation at a Renfe ticket office in Spain within 48 hours.  So this method may gain you time, but not much!

Please look at the reply below. You might need to use a VPN to a country in Europe. 

https://community.eurail.com/train-connections-reservations-47/seat-reservations-error-in-spain-14676?postid=67225#post67225