Based on the prices you mention, I guess you’re trying to book a domestic French TGV reservation. Note unfortunately only a limited number of seats are available for that price, and when those sell out the remaining seats are sold for €20 per passenger.
Check https://travel.b-europe.com/Eurail-GE/en/booking-tgv to see whether €10 tickets are still available. It seems the Interrail reservations portal is currently not able to show those €10-tickets, so there might actually be €10-tickets available by booking through b-europe or SNCF.
Prices in the interrail reservations portal add +€2 booking costs per ticket. When booking through the b-europe website they charge you €4 per booking. You can avoid those costs by buying them through the phone from SNCF (+33 184943635, dial #85 to be helped in English) or buying them at a SNCF ticket machine & ticket desk.
Note the Interrail app has no realtime information on reservation availability, and just mentions the minimum price.