You can book reservations wherever you like. The Interrail website should be the last choice because of the extra fees and unreliability.
There are 2 night trains.
1. Zurich dep. 21:40 via Linz (Austria): book via cd.cz, using Interrail as a discount. It's actually only 1 through sleeper carriage, so no couchettes or seats unless you change in Linz very early in the morning.
However, for this particular date, the CD website doesn't show a through connection, so it may be that the one through sleeper carriage is not running. Ask CD by mail what's happening, they're usually quick at replying.
2. Zurich dep. 19:59 via Leipzig (Germany): book via tickets.oebb.at, using Interrail as a discount. Couchettes and sleepers are fully booked on that date, only seats available (running as EC 459).