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Are you trying to book on a day it actually runs? The Paris - Vienna Nightjet only runs on Monday, Thursday and Saturday towards Paris and Tuesday, Friday and Sunday to Vienna.
The train appears in the OeBB online booking engine. If it isn't bookable, either it is already full, or booking has been blocked (because of engineering work, possibly - it sometimes diverts via Passau, or starts short at Strasbourg.
According to nightjet.com there are only sleeper tickets available for the 30th of September, only seats at the 2nd of October and all three categories (seat/couchette/sleeper) on the 4th of October.
Then a few weeks follow with all categories available, but as of the 21st of October till the end of the month only seats are bookable.
We are aware of an issue with emails from the Forum to Microsoft-based email accounts (hotmail/outlook/live.com email addresses). This is being looked into currently, thanks for your patience meanwhile.