There's no couchette carriage to Prague anymore. You can either book a couchette to Leipzig (in the part to Berlin) and then change to a seat, or book the direct sleeper.
I see ÖBB currently only offer a Single (1-person) compartment online in that train, but not a Double or T3, which looks like a bug on their booking website, since this means a full compartment is available and you should then be able to book any of these options. You can either book by phone (+43 5 1717) or use cd.cz, which does show all options. Add Interrail as a discount. The SBB ticket office (or call centre) should also be able to book this. I found e.g. CZK 1885 for a T3 on 10/2.