Many there are already posted about the problem that actual ItaliaRail.com doesn´t recognise the PassCoverNumbers at the moment the bestway is actually to reserve at a International Ticket Office or you ask the Support team for help :) 

I know there have been several other topics. Those topics were about Italiarail not recognising Pass Cover Numbers. However, since of today Italiarail completely removed the option to make reservations. So before you could still make reservations with older pass numbers. Now you won’t be able to make reservations via Italiarail at all.

So. 3 years ago Trenitalia made it impossible to make reservations on their website. Now Italiarail did the same. The Eurail/Interrail website and app are the only options left… (and the ticket office of course, if you don’t care waiting)