If you are referring to the rail planner app: You can’t book seat reservations there.

You may book with Interrail on their website (not really recommended because there’s a fee of (additional) 2 € per person per train). There are many places to book seat reservations but wich one is the best depends on country / train.

If you are interested in booking a specific train: just ask.

BTW: Not every train does require a mandatory reservation. Sometimes it’s enough to add the train in your rail planner app and take any available seat when boarding.

You can't book seats in the app. It can only forward to some of the websites where you can book.

Here's an overview with ways to book reservations:

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

If that doesn't work for you, then please mention route, date and departure time of any train you'd like to book.