Since June 29th, 2015, eBay requires 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 or updating 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.
In order to resolve this issue, you need to add MPN to your listing templates. Please refer to the following documentation article for a detailed guide: How to Set Up and Work With Product Identifiers for eBay in Linnworks
There is also a section called 'Updating existing listings' which will explain how to deal with live listings that do not yet have MPNs.