All highspeed trains that run in France like TGV, TGV Lyria, Eurostar, Thalys, ICE and AVE have compulsory reservation. Intercity trains needs also a reservation. Also some TER trains to/from Normandy needs a reservation.
The app is not reliable, you better check the official website of Interrail.eu and the Information about trains in one country.
You should have bought the 20 EUR Interrail reservation from a place like SNCF and not a new ticket for your travels.
Interrail can only sell the limited 10 EUR pass holder reservation for TGV. The 20 EUR pass holder seats have to be bound elsewhere.
IN FR its either /or: you MUST have RES-see above for the quota-or it is not possible at all.
FR does not have ´express trains-its TGV and IC.
SNCF would not let me buy a reservation. I was in their ticket office & they were very helpful checking all options, but they insisted I had to buy new tickets and reservations for the train as the Interrail quota was all taken.
There is no Interrail quota on the TGV trains. You should contact Customer Support and ask for a refund of the ticket that the person st SNCF forced you to pay.
Use this form to contact Customer Support and include a copy of the ticket you bought.
https://eurail.zendesk.com/hc/en-001/requests/new