Which trains are you trying to reserve?

You can reserve even via the Website of the Interrail or with luck on the website of the operator directly :)

Direct via app only possible for some countries/trains. In most cases the full info is to be found on the site of EU/Interrail by simply reading the COUNTRY info-there f.e. it will state that in IT and PL you have to go to the ticket-counters!

As far as I know, the app just refers to one or more websites where you can book.