Nej. UNavailable means canNOt be done via that way. Also note there are very few direct trains between the 2 and these are generally also NOT (ikke) avail. for passes. The usual way is via Zagreb-and then local HZ train, due to very low nr of these and high demand must do REServ-at HZ counter in Zag. There are also hourly or even more buses from BusSt=much fatser, but have to pay. Lj-Zag is with REServ optional.

YOU can ask in Lj if they can REServe for Hrvatska, but till now the scarce reports tell us NO can do. Somehow it seems that suddenly about anyone has discovered this remaining single line to the coast and wants to use it

But before I bought the pass, it showed that it would costs 32 euro for a bed on the Night train?

Which Nighttrain?
From Ljubljana to Split is none :)
You can catch the Bratislava - Vienna - Graz - Maribor - Dobova - Zagreb - Split Nighttrain very expensive even with Interrail
Or catch in Zagreb the seasonal (end of June till mid August) Nighttrain from Ossijek/Vinkovici via Zagreb to Split (this one is much cheaper)

This one

This one

Yeah this is the Nighttrain from Bratislava/Vienna  to Split and the prices aren´t correct.
If you still wanna catch the Train you can reserve this train via oebb.at by following these steps (dont use the seat only option)