Venezia to Vienna

Hi travellers, 

To my interrail trip i was going to go from venezia S. Lucia to Vienna Centrale, but when i go to buy the seat reservations it tells me it inst available, i also have seen for some other days but it happens all the  time, if someone could help figure it out I would very much appreciate it. The day I was planing to go from venezia S.Lucia to vienna Centrale was on the 14 of august and the time of departure was at 21:05 and arrival time at 06:05. 

Thanks :)