RENFE made booking pass holder reservations for domestic trains difficult intentionally, including the removal of online booking. They're, well, passenger-hostile. The trains themselves are fine though.

The current known pass holder booking possibilities are listed here:

Reservation should be €6.50 in 2nd and €10 in 1st class.

If this will be your only only journey though, then don't buy a rail pass and just book your journey on the RENFE website.

So I book accommodation in Valencia. Arrive in Barcelona, ring Renfe, or go there in person, if that’s not too much trouble for them, hope the train is not booked out, maybe get a ticket to Valencia, if not we’ve wasted our money booking and planning ahead, and getting a Eurail pass, which seems a bit useless in Spain.

Yes you go to any Spanish train station as soon as you arrive in the country. There should be available seats, don't worry. It's not too much trouble for them, it's their job !

If you're in Germany or Switzerland at some point before, you can also buy seat reservations there. See rvdborgt's link

Yeah Spain isn't a great country for Eurail but that's the fault of RENFE (Spanish national railway company) ! Until a few years everything could be booked online…

If you're only travelling in Spain, a pass is surely not worthwhile

We’re travelling in France and Spain. We have booked to Barcelona, no issue. To tell you the truth I’d just as soon cancel our Valencia Airbnb booking and turn around and go back to France. Not gonna book any more accommodation in Spain if there is the chance we cannot book trains. I hear the Spanish hate tourists anyway. Didn’t know it was such a hassle when we bought the Eurail passes. Didn’t think to do an internet search on “the hassles of using Eurail in Spain”. Well, we live and learn.

This was from a thread 3 months ago and might be helpful? 

I didn't know that Andy is now able to email seat reservations so yes it's a good option if you're worried.

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

Not gonna jump through hoops to go to Spain. I’ll cancel the Spain leg and stay in France. Who would have thought the French treat tourists better than the Spanish. Maybe Eurail should put something in writing on the, “select the best Eurail option” page that Eurail is useless for Spain. 

I can’t figure out how to get the Renfe phone line in English? Guide says press 1 but it doesn’t seem to do anything.

This may be helpful to those traveling who have friends residing in Spain. We wanted to book a high speed train from Marseille / Aix TGV to Barcelona. That obviously wouldn’t work for us since we are currently in France. We have friends that live in the North part of Spain close to a train station. We sent them our itinerary and Eurail passes and they were able to book them on our behalf and send us the digital tickets. Total cost for 4 tickets was 42.50 Euros. (instead of 108 Euros full price per ticket).

We have yet to take the train in two weeks, but we have the digital tickets with seat reservations and barcodes in hand. We tried calling Renfe, tried the Deutch Bahn Office, tried booking directly with Eurail, tried going to the Aix TGV in person… and none of those worked.

This may have been successful since it was a smaller train station and hey got someone helpful. We also send them pictures of our photo identification just in case they needed it (I don’t think they ended up needing it after all)

Hopefully this helps if someone has a trusted connection who can reserve and pay for the tickets on their behalf!