Please note this is a channel-specific article only applicable to eBay.
Since June 29th, 2015, eBay has required specifying product identifiers for single-item and variation listings in certain categories. More information on the Product Identifiers change for eBay can be found here and in our blog.
When creating a new listing or updating an existing listing that does not have a Product Identifier, eBay will return the following error:
Error (21919303): The item specific MPN is missing. Add MPN to this listing, enter a valid value, and then try again.
Error (21919301): The EAN field is missing. Please add EAN to the listing and try again.
This issue is resolved by including MPN in the listing templates, which can be done through Extended Properties. Please see the following documentation article: Extended Properties for Listings. In addition, a section called 'Updating existing listings' explains how to deal with live listings that do not yet have MPNs.
You may also find the article eBay - Different Product Identifiers for different marketplaces helpful in understanding the exact steps.