Hi try to get most of the reservations not via Interrail.eu because it has often bugs or problems. Search here in the community for tutorials to get reservations by other companies. 

For Italy you can book via Austrian Railways ÖBB tickets.oebb.at using one way ticket option and at adult choose as discount Interrail/Eurail Global Pass. 

Reservation costs: 10€ Frecciarossa and EC within Italy, 3€ IC, 11€ 2nd class EC IT-CH. 

UPDATE: I see now, that it is for tomorrow. You can also book at any Trenitalia Ticket Office. 

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105?postid=9201#post9201  

So is this to say, you should not buy a Eurail pass and just book trains on with each trail company separately?

No, that's not what Angelo said. He just pointed out that online booking of pass reservations for Italian day trains is best done via the ÖBB website or at the Trenitalia ticket office.

We suffered a technical glitch around the time of posting and I am sorry that your reservations were affected. We do strive to resolve these asap, but if you are traveling soon, you can reserve it locally at the station or checkout alternative reservation methods that are listed per country over here. This post from the Community also has detailed instructions. Indeed for travel from Naples to Venice, you can make reservations either via OBB or ItaliaRail.