Yeah since SNCF vanished their Interrail Reservation option only struggles i hope you open a case on oui.sncf for the missing Interrail option (they removed it directly on start of the summer season for a “service update”. I assume they wanted to make it harder for Interrailer and earn more money with standard tickets :/
You could go to a International ticket office or a ticket office of SNCF next to you as the Route is very popular it can happen that this Nighttrain is already full :/
Note just Couchette 4pers Compartments for 1st Class, 6 pers Compartments for 2nd Class or seating cars :/