A postal number is mandatory for deliveries to a packing station, otherwise DHL cannot lock the parcel in a compartment.

You can find your postal number here:

• in your DHL customer account under "My data and services" in the "Personal data" section
• in the Post & DHL app under the "More" tab after you have logged in

Further details can be found directly at DHL:
https://www.dhl.de/de/privatkunden/pakete-empfangen/an-einem-abholort-empfangen/packstation.html