While working on the Quickbooks Online API for a client, I ran into an issue where the
Sku field was missing or being ignored from Product (Item) requests and responses.
I found this system strange at first but it is a good way to ensure non-breaking API changes without using a new major version in a separate namespace like some other APIs do.
So to get the Sku included in responses or to be able to include it in your product creation, your requests should include
minorversion=4 as a query parameter.
With that your request URLs should look like this:
https://quickbooks.api.intuit.com/v3/company/<realmID>/item?minorversion=4 # or https://quickbooks.api.intuit.com/v3/company/<realmID>/item/<itemId>?minorversion=4
Hope that helps! This article is mainly meant to be an easily Google-able version of the other resources linked.