Please give us the train route, date and departure time. It could be unavailable for plenty of reasons.
Have a look at https://www.seat61.com/interrail-and-eurail-reservations.htm
This is for the Amsterdam Centraal - Rotterdam Centraal route on 09/02, departing at 9:08 am, arriving at 9:47 am.
In fact, I see this for all the routes, but looking at the website you sent me, it's normal because you don't need to reserve a seat.
Thank you for your reply !
Yes reservations do not exist in Belgium and the Netherlands (except on Eurostar trains + IC/ICE to Germany). Simply hop on and sit wherever you want.
Enjoy the trip! :)