I can't find a Prem's rate for my itinerary
1.
Your segment does not appear in the list of PREM'S reductions.
SNCF is constantly striving to set up segments and rates that best correspond to customer expectations. The offer must satisfy the majority of the customers, keeping in mind that it is difficult to satisfy everyone at once.
2.
Your segment appears in the list of PREM’S reductions but the site does not offer any PREM’S.
If you are experiencing difficulties reserving the promotional areas:

Click on the "Reservations" tab from the home page:
The Prem's rate is sold up to the limit of available seats, starting from 22 euros for a TGV segment and 17€ for a Téoz segment. If you cannot find a Prem's rate from 17 or 22 euros, this means that this rate is sold out.
You can also benefit from Prem’s rates from other SNCF sales outlets:
In France: at counters in all stations, in our RAIL EUROPE sales outlets or in authorised travel agencies.