You can't book pass reservations on the CFR website.

There are 3 night trains from Budapest to Bucharest and only 1 of them can be booked online: train 347 leaving Budapest Keleti at 22:45, using tickets.oebb.at. The other 2 can only be booked in person, either at a ticket office or by phone (e.g. by DB or ÖBB).

Actually, you can book the night trains over the Hungarian Train website (https://www.mavcsoport.hu/en). I am not sure about adding the interrail pass though, as I opted to travel this part without one.