Some routes are only available in paper form, esp. in Eastern Europe. Otherwise since you seem to live in Romania, feel free to book at a ticket counter. That should work.

Otherwise mention which night train you're taking and we'll advise. Only the Dacia (stopping very early in Budapest, terminating in Vienna) is available online on the ÖBB website.

I tried to book at a ticket counter, but i couldn't book only the seat reservation, i would have to book the entire ticket...

The night trains Bucharest-Budapest should be bookable online on https://jegy.mav.hu/

Instructions are here. You should receive your reservation by e-mail.

If that doesn't work for you then please mention date and departure time.

If you were at a ticket counter in Romania and they told you they couldn't book a pass reservation, then the staff should look in their documentation or ask their helpdesk. They can definitely book a pass reservation.

It works!! Thank you so much!!

Also, if you know where I can book night train from Vienna to Bucharest, that will be great, too. Again, thank you so much!

Should also work via MÁV, or also via tickets.oebb.at: add the Interrail/Eurail discount and do NOT check "Seat reservation only”.

MÁV only sells tickets/reservations for international journeys to or from Hungary...

Exactly. I found the ticket on oebb, but i don't know how to book only the seat reservation. 

Add the Interrail/Eurail discount and do NOT check "Seat reservation only”.

If that doesn't work for you, then please mention your travel date.

It worked!!!! Thanks again!