Error 21919301: The field is missing. Please add to the listing and try again.

This error is triggered when the listing category requires a specific type of GTIN, such as an EAN, UPC, or ISBN. Based on the missing GTIN type that is called out in the error message, you must supply this GTIN value through the ProductListingDetails.EAN, ProductListingDetails.ISBN, or ProductListingDetails.UPC fields. For a multiple-variation listing, these GTIN values would be specified through the VariationProductListingDetails.EAN field, the VariationProductListingDetails.ISBN field, or the VariationProductListingDetails.UPC field. The getItemAspectsForCategory method can be used to see which GTIN type(s) are required for a category.