You cannot make reservations in the app. It can only forward to some of the websites where you can book, including the Interrail website, which unfortunately is not the most reliable or cheapest website to book.

An overview of other ways to book can be found here:

If you still have questions after reading that, then please specify route, date and departure time of the train(s) you'd like to book.

For trains in Italy, you can also book at a Trenitalia ticket office.

In most cases if done online its via a secured link with crd.cd. If you do not have such a thing (pretty common for young people) than you have to find other ways-most often that is still go inline and wait for the counter to help you out. As anyone did like that not that long ago, even also in IT.