What website?

What accommodation are you trying to book?

Highly likely beds are sold out (a week or so ago they told that by then 95% in summer were booked) OR that again there are some works along the line (in NL) that this train does not run its full course.

Try from a nearby stop in DE, like Dusseldorf or KÖLn-via oebb.at. Then travel to there in the afternoon and double check times again the day before.

Thankyou both for your replies.

I finally got through the the support line and she said they were sold out of sleepers and that it’s a holiday weekend there! However the OBB Nightjet website still offers sleepers, which is confusing.

Thanks mcadv for you alternative suggestion. Might try that.

You can reserve this Nightjet even on Oebb.at with Interrail 

Just have a look in this post 

When I looked this up on OBB 10 hours ago when you first posted the question there were seats and couchette spaces still available but not private sleeping or sole use of couchette rooms.