It should not be possible to reserve online via Interrail/Eurail or you get the reservation via postal mail. Can you gove us a screen shot about the error message?

Bookings should be possible at Ticket desks in Croatia and maybe at ticket desks of train companies in middle europe like DB, CD or ÖBB.

Yes, we are trying to book online. 

And then same message from Karlovac to Budapest minus the message in blue, just saying it’s possibly all sold out 

THIS trains-and all trains on the line Split-Zagreb, can ONLY be RES on he spot when in HRvatska-by a human being working behind a counter.

I guess you aim for the seasonal direct train to Bp:  be flexible, there is a very big chance it has been fully booked by normal pax.

@mcadv thank you, not sure what Bp means, but are you saying both of the trains I mentioned (split to Karlovac and Karlovac to Budapest) have to be booked in person?? Is there a way to see if they’re already sold out on your end? 

The messages you are seeing are generic IR planner statements when they cannot access the providers reservation tools - it covers both full services and those that need a different reservation method - in country or an alternative reservation service are the usual.

BTW (by the way) bp is shorthand for Budapest.