What is the postcode

A postcode, also known as a postal code, Eircode, pin or zip code is a number that identifies a specific region served by the mail service in a country. Its part of an address such as:

234 Kengsinton Road, 54372, Charlotte ,North Carolina, U.S.A

https://en.m.wikipedia.org/wiki/Postal_code

If your country does not use post codes, then you probably have the wrong country selected for your Apple ID.

Or you are using the wrong Apple online store. The US Apple Online Store will only ship to US states and certain US territories to comply with US export of technology laws.

Please see the section, "Shipping Policies" in this Apple article:

Shipping & Pickup - Shopping Help - Apple