Please give us the full details so we can help 

We need at least a travel date.

Can be booked via Italiarail.com with a Railpass 

The question is on which date? As not on every date this train goes via Florence Some of the depatures from Sicily go via the old more scenic Route via La Spezia and Genoa 

June 21. A later departure. I tried the online and only seats were available 

It's still too early for 21 June. Trenitalia already has the timetable loaded but nobody else, so wait a bit until it becomes available e.g. on the Italiarail website.

Also note that Sicily is not a train station. The sleeper starts in Siracusa and Palermo, with multiple stops until it goes onto the ferry.

Thanks for the info. I tried for the station out of Catina. I’ll look for Siracusa. It’s closer to our hotel. What station do I need to choose for Florence?

Doesnt matter as the train departs from both stations Siracusa is the starting station of this train.