seat reservations

I am looking to travel from Ljubljana to Split in July, I have the ticket on the Eurail app but when I try to reserve a seat it says that “No price results were found. Pass holder reservations may be sold out for this connection, but there are other ways to check for available seats—more info below our timetable.”  and that I can’t get on the train without a reservation for any of the times/days. I have tried looking on other websites to make the reservation but none are coming up. Any ideas on how to look into it? Reservations only opened up yesterday and I need to get that train as my flight back home is from Split.