Hej pa dej. Forget APP. Also forget It-rail=its a private person in Boston/US who makes some sidemoney by helping people doing it in italiano, but he has stopped doing this last week.
The main SJ in Italy is TrenItalia
Its just by coincidence that I am now a min. or so earlier as fröken Anna, ocksa fra Sverige, she links to the page by seewulf on howtodo REServasjoner by yourself via local sites. For IT its best to use a byway via oebb.at (austrian rail).
You probably also have to make REServ. for X2000 till KÖpenhamn and in summer for DSB direkt till Hamburg. Och trevlig resan!
Hej!
The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:
https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105
If you, after having looked at the guide, have questions about how to make specific reservation, please create a new topic with your travel details (departure date, time and route) and you will get advice.