No need to do reservations on regional trains, you can’t anyway.

Simply board the train and find a seat :)

That is not needed to make reservations. In fact, it's best not to activate your pass or any travel day in advance since you never know what will happen and you can only cancel activation until 23:59 before validity starts. if something unexpected happens after that, e.g. you wake up ill, then it will be too late to cancel and you'll loose travel days.

thanks for your answers. It doesn’t show the two trains that need reservation on the screenshot. But i found out the problem. I needed to buy one reservation directly with SNCB. I don’t know why but it is how it is..

Yes SNCB sell TGV reservations within France for 10 or 20€ (4€ booking fee per order). Interrail.eu does too for 10 or 20€ (+ 2€ booking fee per person per train).

The expensive cross-country TGV into Spain can only be booked on interrail.eu (25-35€) but there’s always the alternative route by the coast (Portbou)

Let us know if you have any questions

These 2 pages are very helpful :

https://www.seat61.com/interrail-and-eurail-reservations.htm