You can do it online at oebb.at for the Nighttrain but you need to find a way to print it i assume you are in Munich? 

Thank you. I tried, but no matter which ticket I choose, it says that there is no ticket left.  But my friend whose enterance country is Germany can find a place when he chooses it from the application.  

The train has already left Munich but from Rosenheim there are still free seats, which means that with 99.9% certainty there were also free seats left from Munich, which the ticket office should have been able to book. There is no requirement to book via the rail planner app; in fact, the ticket office was obliged to sell you the reservation you requested. Was this at München Hbf? Or at another station? Please let us know so @Nanja can (hopefully) feed this information back to the operator.

In the last instance, the train staff can also sell a reservation for any free seats left.

Seat reservation availability does not depend on the country of residence. Unfortunately, you didn’t send a screenshot, so it's hard to determine what the problem was.

Strictly speaking, the rail planner app cannot book reservations. All it does is direct you to websites where you can book.

this was in frankfurt hbf.  then I came to Munich Hbf.  They also didn't make a reservation.  I will use the train, which does not require a reservation, and therefore  I will wait for hours at stations.

@rvdborgt 

The Nightjet closes pretty early for reservations at the ticket offices of DB :/  I just returned by Nightjet from Vienna to Berlin and the Sleeper Coach conductor had a smilar problem. Passengers booked last minute at the ticket office in Vienna HBF (he haven´t seen them on his passengers list) and he was to sell this berths once more direct on the train. Luckily the passengers (from the ticket office) were early enough at the train to say it´s their berths. To avoid these problems DB stops to sell reservation quite early :/

Thanks for the info. Any idea how early? It must be at least ~4 hours, since already the Frankfurt ticket office couldn't book. I'm not sure if this early closing solves more problems than it causes and the ticket office should anyway be able to give an explanation.

@rvdborgt  I just asked and opened a internal case within the DB Network as our Service Team could say that it´s closed but couldn´t say when it closes. :)
To big company with to long ways to get information. And iam the deadend for Informations as iam not in “DB Vertrieb” sales :/
Iam just a technican for the Railnetwork :D

Hi Serife, as mentioned seat availability doesn't depend on country of residence. Can you please provide more information on where you have seen that it didn't work because of your country of residence? It would be really helpful if you can provide us with a screenshot too, so that we can investigate.