Please specify date and departure time.

There should also be plenty of reservation-free options.

I face the exact same problem. Trying to book seat reservations for two different trains in France and Italy since yesterday evening. "Prices unavailable".... My friends booked yesterday and they all succeeded. Don't know how to get this d*mn seat reservation...

Advice is hard if you don't specify your trains: origin, destination, date, departure time.

@Izzy like rvdborgt says, without further Information we can’t help. But a small advice about Italian Reservations, you can book it online via tickets.oebb.at - one way ticket and enter at 1x adult the discount code Interrail/Eurail Global Pass. 

I need seat reservations for:

Venezia - Torino   and

Narbonne - Bordeaux

My friends have already booked so I would really like to travel with them instead of taking another connection etc.…

Venezia - Torino: book via ÖBB, as @Angelo wrote. That's also cheaper than via Interrail.

Barcelona - Narbonne - Bordeaux: call SNCF. Press #85 for English, no booking fees, reservations are sent via e-mail. If you know where your friends are sitting, you can request a seat in the neighbourhood.

Ok, first of all thank you for helping so quickly.

I just tried to book at ÖBB but it says no tickets available for this train, in fact for all the trains from Venezia to Torino at this day...

Please do not use the "Seat only” option. It does not work for reservations in Italy.

Here's what Angelo wrote: “one way ticket and enter at 1x adult the discount code Interrail/Eurail Global Pass.”

Alternatively, check these steps (with screenshots).

Okayyyy I´m so happy it worked, I have a reservation for the train from Venezia to Torino!!

Tomorrow I´m gonna call SNCF then