Hello @debbie,

In order to book a seat reservation for a domestic French train, you can use an alternative booking website: Book your international train tickets (b-europe.com). Otherwise, you can try to turn on "No seat reservations reservations required" filter on the Eurail website's reservation page. By using this way, you can ride some trains which do not require any reservation.

I hope I explained it to you clearly. Have a nice weekend!