There are problems with the Interrail Reservation service for trains in Italy. Which train do you take?
The best way to solve this is when you get in Italy to get changed the reservation at the ticket office or at the last minute desk in bigger stations. Last way is to ask before entering the train the train staff if it is ok to get a seat in 2nd class without reservation.
You can also ask Trenitalia Costumer Service or write on Social Media (Facebook or Twitter) a private massage to @LeFreccie.
The next time book your reservation for Italian trains via Austrian Railways tickets.oebb.at - one way ticket - discount: Interrail/Eurail Global Pass