Which train? The nighttrain Nightjet? Or with change in Venezia Mestre with Regionale Veloce or Freccie and Railjet?

You can book almost all reservations for trains in Italy and Austria via tickets.oebb.at (Austrian Railways) using this steps (thanks to Seewulf :)

How to get Reservations | Community (eurail.com) 

Depends on which train. It may also simply be too early now.

Note that the daytime trains IT-AT/DE have a special extra surcharge of 10€-and the app/system seems still not able to handle this.

Also check a few days before/after, it may also very well be that there are works=trains cancelled or rerouted-I somehow remember there are even longer time works on the border sector IT-AT, but Ie forgotten around what term.

AND be a little flexible-that is one of the wonders of this Pass; you can also take any of the trains to München as far as Innsbruck and change there-in a nice RailJet, without need to REServe..

Due to works on the tracks between Udine and Villach via Tarvisio, no trains will travel on this line in July. 

• Nightjet Vienna/Munich-Rome/Milan do not run at all. 
• Nightjet Livorno-Vienna do not run at all. 
• Railjet Vienna-Venice run between Vienna and Villach and with bus Villach-Venice
• ÖBB IC-Bus (Villach-Venice/Trieste) run 
• Nightjet Venice-Vienna/Munich run via Ljubljana. 

Please specify the exact trains you want to take (origin, destination, departure time per train). Or post a screenshot of the timetable.