In Spain, many trains have mandatory reservations and they are often checked before boarding. The reservation needs to be for that specific train, not for any other.

It may be possible to change the reservation if there are still seats available.

Where did you book the reservations? Which train would you like to book? (route, date, departure time)

Grazie mille della risposta così veloce! Io ho prenotato il treno da Cordoba a Sevilla-S Justa AVE 02140 delle ore 15:49 del 23/02/24 , mentre la mia compagna di viaggio il treno con la stessa destinazione ma con partenza prevista alle 16:49. In caso i posti sul mio non fossero finiti, come si può cambiare il suo biglietto ? I posti li ho prenotati da questo sito “eurail” utilizzando l’interrail global pass 

There is still availability in all classes on both those trains. Ask customer service to change one of the reservations:

https://eurail.zendesk.com/hc/it/requests/new

And explain that you didn't manage to book onto the same train because of an error.

For other trains, the best is to book all persons at the same time in the same booking, so you will be seated together:

https://www.interrail.eu/it/support/reservations/how-do-i-make-seat-reservations-together

For other countries than Spain, check first this page on where to book reservations:

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