Could you please post details of the train(s) you're trying to book? (origin, destination, date, departure time)

The first I have to take is on April the 3rd.
TGV 5304 

“Le Mans” to “Nîme”

10h31 AM to 2h56 PM

You can book reservation directly at SNCF Ticket Counter or SNCF ticket machines. 

Just to be sure, if I go to a ticket counter, I’ll have to possibility to use my interail pass to get a ticket at a lower price right ? thanks anyway

No, you need a reservation. You have a Ticket (your Interrail).

For TGVs in France it is 10€ (only few seats) or 20€ (all other). I heard somewhere, that people in France get reservations to leave France with Interrail for free. But I’m not sure. 

Another advantage of the SNCF ticket office is that you don't pay Interrail's booking fee (2€/seat).

Thanks a lot guys

What happens if train is full?

Then they wont sell reservations anymore (Sold Out)
As on a TGV every passenger need a seat :)

Also-this route needs 2 (TWO!) TGV-trains and also a change in Paris from Montparnasse to Lyon-by metro=pay ticket yourself. I guess that is also a cause why cannot do in 1 go.

OR-you can go via the classic route without TGV-via Nantes-Bordeaux-Toulouse and even without any REServ needed. Will take longer and have to very carefully check timings-this is SNCF after all, not noted for a frequent service (but yes for frequent strikes). Use SNCF site to plan and cut up in segments if you would like to check this