Italo trains are a private company, not included in the pass schedule.
It’s an official holiday, the 8th June. Busy weekend but still a month away, normally there should be availability.
But the train company itself didn’t release the ticket sale yet. Normal tickets cannot be bought as well. Not a single train Venezia-Rome can be booked, which means there’s an uncertainty in the timetable because of engineering works probably.
Just try a bit later, in a week or so, sales will open when the timetable is definite.
If nothing shows up, take the regional train to Bologna at 16:40 (no seat reservation needed), and reserve a Freccia-train from Bologna to Rome.