Most likely its just yet another bug/wrong posting in the app of the pass.
Hi Sara97pt,
if you’d like to get a reliable answer, it’s no good idea to hide all necessary information.
There is no single train from Ljubljana to Villach (surely availability depends on the specific train), it makes a difference if you’d need a reservation just for you or for a greater number of people and it makes a difference if you’ll travel 1st or 2nd class.
To summarise it: Every train on this date has availability at least for 1 person in any of the classes and none is sold out and all of them are bookable online.
So it seems that you’re just doing something wrong when trying to reserve. So it would be also necessary to know how and where you did try to reserve. Easiest way ist the website of austrian railways.