You are trying to book reservations for the Zagreb-Split train or which one?
I think that one is not possible to book online

Thanks for your reply and help! Yes I want to book from Zagreb to split but it says €0 when I try on eurail and when I try the OEBB website it says ticket not available. If I can’t book online can I reserve at the station? 
thank you so much :)

Youre too early-reckon on 3 month advacne at the most-2 is also possible. The common mistake of about 99,99% of those enthusiast newbees.

Its also not a matter of now or only 5 mins before dep-that latter is also unwise. You probably will not come rushing off a ferry to catch a train in the next 15 mins? This age-old trick still works: (from times that normals did not even know about www): on ARRival-just spend 5-10 mins more at station, search desk/counter and have it doen there. In that past you also had to change money (HR gets € next yr-now its Kuna), find an office to book accom etc.

Spl has a very few daytime trains to Zagreb and in season ev 2nd day an overnite/internal via Zag to Osijek and on some days/weekends a direct via Zag to Wien/Bratislava. RegioJet=the CZ yellow and quite nice private train also serves it, but does not accept passes on this route.

Change mindset; In HR -if people think about transit-it will be bus. Ev 30 mins to Zag in much shorter time as train=vlak/voz

I want to reserve seats for Split-Zagreb in July. Contacted HZPP (Croatia national railways) and they told me that it’s not possible to reserve seats in Croatia online, only possible at ticket office of railway stations in Croatia.  

Correct. The best is usually to go to the ticket office when you first arrive in Croatia.