Which date?

You can´t use the TGV without valid reservation You risk a quite high penalty fare :/

The interrail System sometimes have errors with french trains :/
You may request a PassCoverNumber from Customer Service and book then via
B- Europe https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

Or you call directly french rail and book via phone

Thank you for rapid answer. Ive tried to book via b-europe and it wont let me do it because i cant find my pass number? I have a mobile pass not physical. 

You can call SNCF and make a reservation if there are seats avaliable. No need of Pass Cover Number. 

https://www.sncf.com/en/customer-service/contact-us/telephone

 Press #85 for English, no booking fees, reservations are sent via e-mail

For P_> N you have this choice:

1 a very few through TGV by day

2.hourly TGV to Marseille, then UNreserved local TER train (over the very same rails) onward

  TGV MUST be REServed and in fact there are now gates in Paris-Lyon that you cannot pass without a REServ precise for that train/day/time.

3.Overnight train (from Austerlitz) with seats (1st cl style) and couchette=simple lie down bunk.

I am sure you will find something if you can be a little flexible-though as summer comes: the FRench have also what they call ´the days of big departures´=last+1st on july and aug (and 31/8 coming back to P) and for that you better replan. Its hellish blue murder. Also a bit less around 14/7=that national holiday.