Interrail publish a guide - https://www.interrail.eu/en/plan-your-trip/reservations/how-do-i-book-my-reservations - which provides other options for how to obtain a reservation. Their own system cannot issue reservations for all trains - and when it can you can often find reservations cheaper by going elsewhere.
Depending on when exactly you are looking to travel I strongly suspect that the issue is you are trying to make a reservation too far in the future. Depending how far in advance you are looking you might have better luck going direct to the train operator, even once opened reservations are not always immediately available at interrail.eu.
When reservations have opened some options to make them are:
For SNCF trains (including TGV) you can't make a reservation direct with SNCF using their own website. You need to visit a station or phone up. Their phone number is +33 1 84 94 3635 and they are open 0800-2000 7 days a week (https://www.sncf.com/en/customer-service/contact-us/telephone). If you press "#85" once the automated voice starts you can change the language to English. Reservations can be emailed to you.
You can also make reservations for TGV trains online through B-Europe - https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish - though this only works for legs completely within France. If you have a mobile pass you will need to generate a pass cover number using https://www.interrail.eu/en/book-reservations#/generatePassCoverNumber
You can normally also use the Interrail.eu reservations service. Reservations on international trains from France to Germany can also normally be obtained through Deutsche Bahn. You may though need to phone or visit a station if their website (https://reiseauskunft.bahn.de/bin/query.exe/en?&rit=yes) does not co-operate.
Interrail reservations for trains in Spain are infamously annoying. They cannot be obtained from Interrail.eu or easily from any online source. You can make them locally at ticket offices within Spain. You can also make them by phoning RENFE at +34 91 232 03 20 - however you must collect the reservation within 72 hours of calling or it will be voided.
It might therefore be worth considering buying a standard ticket which can be done easily online. This would also give you a choice of operator, in Spain only RENFE accept interrail but Ouigo (https://www.ouigo.com/es/en) also run trains from Madrid to Valencia which might be useful depending on your route.