Can't book Ljubljana - Split (Night train), prices unavailable

There is no night train from Ljubljana to Split.

And what you are trying to book (screenshot) is a night train from Dobova that goes already to Split (!) with a change at Zagreb to an other nighttrain to Split.

The first one can be reserved online (even for Dobova to Split).

The second one can only be reserved at croatian ticket counters. 

I don’t understand your problem at all. 

You did write “I am going to travel from Ljubljana - Split on the night train” but you already did know that there is no night train from Ljubljana to Split and that you have to change trains?

It doesn’t make any sense to try to book a seat reservation from Dobova to Zagreb for the train above as it is the same price as if you’d reserve it up to Split. So why should you want to do this?

And: If ypu know how to reserve the first train, why would you reserve the second one, too? Then you’d need the supplements for two trains (and they aren’t cheaper if you reserve it for a shorter trip).

As I wrote before: For the train from Zagreb to Split at 1.10 you need to buy the reservation from a croatian ticket counter. In fact this means that you won’t get a reservation for this train if you are not in Croatia before. 

But to repeat it: You should opt for the first OR the second train, not both of them. The first one is leaving Zagreb at 1.05 and is going to Split, too. And for this train you are able to buy interrail reservations by ÖBB website (you did write that you already know this).

Maybe the problem is that you didn’t understand how much the price for this train is. You did write “i could book a ticket + sleeping bunks for 160€” - 164 € is the price for a sleeper compartment with interrail, 159,90 € is the price (at the moment!) without interrail. As there is still a cheap “Sparschiene” ticket available, the system would not give you an interrail reservation as first option because it’s more expensive. 

Does this help?