You must not use seat only but book a normal ticket with Interrail/Eurail as discount card.

Hi Anna, Thank you!!

Hi Anna, 

Do you know how I can book a sleeper cabin? All I can book now is a regular seat..

Just as AnnaB described. There's one berth in a 2-person compartment still available on that day:

How do I get there? When I go to the OEBB and search for this train, fill out the Interrail discount card like Anna suggested and search for tickets, the only option I get is the option below. I cannot choose a sleeper berth. What am I doing wrong?

As I said, there's ONE berth available but it looks like you're booking for 2. Couchettes are also full so then you can only book seats.

Ah my bad, I thought it was a berth for two people. Thanks for the fast answering!!

I am looking to book NightJet Sparschiene Seating Carriage 2nd class at 15-04-2024 20:29 from Stuttgart. Is it possible to book this with Interrail discount? How does that work? 

You add the Interrail/Eurail discount on www.nightjet.com.