There are only a few options to get seat reservation in Spain, none of those option is reservable online.

Read more here:

Most people get their reservations at the ticket counter in Spain.

Thank you! I guess then we need to request refund with Eurail. 

No, that doesn’t need to be. You can use the pass in Spain. The pass still functions as your ticket!

You just need to get your reservations as soon as you arrive. You should NOT be scared of full trains, there plenty of trains between the cities you mentioned. Most people still find a place (except on busy periods, like around Easter or Christmas).

When do you plan to travel?

More information about reservation fees (and the cost) on the trains in Spain, you can find here: https://www.eurail.com/en/plan-your-trip/about-reservations/reservation-fees/domestic-train-reservation-fees

Most people reserve all their trains in one go at the ticket counter, as soon as they arrive in Spain.

With a Renfe Spain Pass, it should be possible to reserve your trains online, btw. It might be better value than a Eurail pass, or a Eurail one country pass.  https://www.renfe.com/es/en/viajar/prepare-your-trip/abonos-ave-y-largadistancia/renfe-spain-pass

And, @anonymous72 on shorter distances, you can travel for free (or a very minor fee), check this post from @Yorkie 

Hi did you end up getting a refund. I have the same issue. Uncertainty of getting a seat in May is causing us stress. I saw they will not give a full refund - only 85% - what a disgrace!!

You can use the Eurail Pass in other countries. Spain is overhyped. TGV Paris-Barcelona is possible online and for one leg like Bercelona-Madrid (I don't know were you like to go) you could make normal ticket and don't use the pass. In Spain on the main high speed lines there are 3 companies, starting with tickets from 19 EURO.