Rome2Rio is often inaccurate or incomplete. ManinSeat61;is great for detailed explanations of everything about trains.
But when I simply want to find the route by train between 2 locations in Europe, I like the German train site, bahn.de. It will show you routes and schedules in all the different European countries.
In this case, you will see that there are various routes possible, with from one to 4 train changes. But the simplest is the one ChristineH mentioned above—- with a single change at Villach. LeVe Venice at 9:46 am and arrive in Salzburg at 14:48, a perfect time to checkin at your hotel.
Once you find a route you like, go to the national train site for the trains involved to confirm the details and buy tickets. In this case, that would be the Austrian train website OEBB.