The best way to make reservations for French trains is to call SNCF. 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

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new post, and you will get advice. 

From C to T:forget TGV_use local TER train-ev 1 or 2 hrs and NO need at all to RES-but as for now these are quite old ex Corail cars-due to get brandnew stock next yr.

You can even do it on the local bus- via St Tropez! gape at the starry models! get struck in endless carjams!- for 3€.

N->B is also quite possible-but slower, more changes, but more scenic route, on local trains-as has been told here a few 100 times now. Even the app has a function for ¨trains with NO Reserv´

If that is too humbly for you: it seems you are in FR for some time-so pop into station counter when you arrive and let them do it for you-after all thats their job. (INTERNat TGV cannot be done easily via other means)