UPDATE: The seat reservations for domestic Spanish trains run normally on the Interrail/Eurail reservation pages; since at least a week ago.

That's about time, although still a long way to go. Much as I love Spain, the absence of an easy way to reserve seats, the cost, and the general lack of availability, has been a big disappointment....it's actually made my Interrail pass a bit pointless as it was impossible booking the various trips I'd hoped on my wider European trip. My advice to travellers that plan to buy a multi day pass and use it a lot in Spain...don't bother! Works great in every other country we visited but it's hopeless in Spain.

Hi Sara, It still doesnot work.

Despite what Mukhammad wrote: if you still find any issue for the Spanish trains’ reservations via our seat reservation page, please let our customer service team know about it via https://eurail.zendesk.com/hc/en-001/requests/new. With the compilation of all incorrect data on their database, it will be more sufficient for the relevant team to work on it. Thank you! 

Well I did, This was the answer:

I can understand your situation, however, railway companies have their own ways set in place how to book seat reservations for pass holders which is different for every carrier, and in this case, Spain and Portugal are one of them. Please follow this link to check everything about Spain by train.
 
You can also try contacting Renfe phone sales through their contact number (+34 91 232 03 20).
You can book your reservations for trains in Spain in the following ways:
 

• Locally at the train station
  • Find a list of long-distance train stations here. 
     
• Pre-reserve seats by calling Renfe phone sales: +34 91 232 03 20
  • Pre-reserving a seat is possible up to 24 hours before a train's departure. You will receive a PNR code which you must use to pick up and pay for your reservation at a local station ticket office, making sure to show your Eurail/Interrail Pass. You must collect your reservation within 72 hours. After this time the pre-reservation will expire.
  • Please note that a pre-reservation is not the same as a reservation. It only holds a seat for you for 72 hours.

 
I hope to inform you well.

Nooo. This doesnot help. Renfe want to expand in EU with connections into France. Please first make the functionality of IT-systems before you go further into EU.
Spain makes travelling by train in EU very difficult and helps people to get into the plain.

And how should I do this traveling somewhere in Europe: Pre-reserving a seat is possible up to 24 hours before a train's departure. You will receive a PNR code which you must use to pick up and pay for your reservation at a local station ticket office, making sure to show your Eurail/Interrail Pass. You must collect your reservation within 72 hours. After this time the pre-reservation will expire.
 

I’m trying to book the exact same train journey but a different day and have the same problem but if I put Cadiz (the stop after) as the destination there are seats available. 

Today, June 22, 2024, I could make seat reservations through Interrail for an AVE train from Palencia to Oviedo for €15 (1 class). Three weeks ago I wasn’t able to book any seat reservations for Spain online. At the station in San Sebastián I was charged €44,75 for a seat on a train scheduled for two days later. So if Interrail has now solved the problem I am happy for for all you folks who will be traveling in the future.