Hi did you try to book diretly via austrian railways that is the company of this Nightjet service?
visit tickets.oebb.at and click on one way ticket and at adult add the discount Interrail/Eurail Global Pass.
Reservation starts at 14€ for a seat. Maybe some categories are sold out.
You didn't say which train(s), but if you're trying to book the direct night train at 20:17, it's better to book it via ÖBB. Do NOT use the "Seat only” option but add the "Interrail / Eurail” discount to each person and then look for normal tickets. Seats are 14€; couchette or sleeper more of course.
If you're trying to book another connection, then specify which train(s) exactly, with departure times.