reserving a seat for jorneys that it is required

You can’t make reservations directly in the app, but it will forward you to a website where you can - including the Interrail website.

However, there are often better places to make reservations. There’s a good list here: https://www.seat61.com/interrail-and-eurail-reservations.htm

if you need more specific help please mention the specific journey and time you are trying to reserve. 

I want to research night sleeper from Budapest to Krakow on Sat 15Th August 26 leaving at 7.30pm getting around 7am to Krackow.  I have my ticket in my pass but wasn’t able to add the seat reservation.  How do I reserve a seat if I have my ticket book showing as qr code but not allowed me to book a seat

Seats Budapest-Kraków can be booked on cd.cz, with "Reservation only”. Price is CZK 74. The sleeper carriage is fully booked on 15/8.