Ola.

Just search: there are extensive manuals on main site and there is ALSO a much more handy overview of ALTernativos by seewulf=giving more possibilities and often for lower price. It differs per country, it is not a systema universale.

Here is a guide on how to make reservations.

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

Hola! Añadí un viaje en España a mi pase, el cual requiere reserva pero no puedo realizarla antes del día del viaje! ¿Es posible que no pueda tomar el tren que añadí en el pase por no presentarme a la ventanilla días antes ? Gracias

Please do not activate your pass or a travel day in advance, certainly not if you need a reservation. There's no need to activate in advance.

Here are the known ways of booking reservations for Spanish domestic trains:

BUT RENFE trenes are often fully booked (or at least they say that) a few days before-certainly on busy traveldays.

BUT; there are various reports here that RENFE wants yo to show activated passday-and even tren, before they will even start up computer to make that RES.

I was also thoroughly checked (paper continu pass) by staff who first had to call supervisor to howtodo it in the small office in MAD-aeropuerto. As if even there they had never ever seen it before.