This night train does not have sleeper carriages, only couchettes and seats.
Unfortunately, the Interrail/Eurail websites have a long-standing issue of not showing what's a seat and what's a couchette, which is apparently difficult to fix.
You can also book this train on https://www.b-europe.com/EN/Booking/Pass (booking fee €4 per order) or www.raileurope.com (booking fee €2 per person and train, like Eurail). Both websites will show the difference between a couchette and a seat. On Rail Europe, always change currency to euro because they have a 5% exchange fee for other currencies.
In general, use this page as a guide to reservations:
https://www.seat61.com/interrail-and-eurail-reservations.htm
It will often suggest better and cheaper places to book instead of Eurail/Interrail.