Could you please mention your travel dates so people can check what's going on? For Berlin-Rotterdam departure time can also be relevant.

In any case, IC 60403 consists of the seating coaches attached to NJ 403. If you want to book a sleeper or a couchette, then you need to book in NJ 403. Additionally, if you don't find availability to Zurich, then try booking until Basel SBB, because half of that train remains in Basel. Booking this night train is best done on tickets.oebb.at.

@rvdborgt 

I plan to depart from Rotterdam on the 7th of July arriving in Zurich on the 8th. 

The departure time I had looked at before was 20:05, with an arrival time of 08:05 (the follow-up train departs at 08:33) 

I planned to depart from Berlin at 00:22 on the 27th of July, arriving in Rotterdam the next morning at 09:25

So if I understand correctly I need to swap this route out for taking a night jet train and I should be able to book a sleeper? Thanks a lot in advance :) 

Regards,
Julian

On 7 July, there's still availability on Utrecht - Zurich, for sleepers and couchettes. There's even still a Sparschiene available for €89.90 (pass reservation is €74).

Berlin - Utrecht can be booked on bahn.com (choose Seat only / Nur Sitzplatz). Note this is not a sleeper but a regular ICE to Duisburg, where you change to the night train coming from Zurich (but then the seating coaches).

Would you be able to share a link to where you found that sleeper ticket? I literally can’ t find it 

On the Nightjet website, make sure to select interrail discount under traveller details or just book the Sparschiene ticket (without interrail as discount then).