Alright reservations for S(pain)... I'll try to help.
So both interrail.eu and happyrail aren't able to book this train for whatever reason. Could be an issue with the Spanish booking system. Could be solved quickly... or not.
(I also remember Interrail only selling 1st class Euromed reservations? Not sure if that's still valid.)
Looking at seat maps on RENFE (deep in the booking process) following trains have availability:
- Girona - Barcelona-Sants 08:09 - 09:40 -> reservation-free (yay!)
- Euromed Barcelona-Sants - Alicante 10:15 - 15:40 -> 125 seats left in 2nd class = plenty
Book it as soon as you arrive in Spain. Portbou ticket counter should sell it to you. Also possible in Germany (DB counters).
That Euromed stops in Girona beforehand so do ask staff if there's space. Looking at the website, Girona-Barcelona seems fully booked/unavailable.
Feel free to ask anything.