The UPC field is missing. Please add UPC to the listing and try again

This error means that the required UPC value was not specified for Products you wanted to list/revise/relist.

eBay requires providing Catalog IDs for Simple and Variational Products in certain categories. To solve the issue, navigate to eBay Global Settings (eBay > Configuration > Settings > Main > eBay Catalog Identifiers). There you should select the Magento Attribute that contains a valid UPC value for your Simple, Configurable, and Grouped Products:

If your products do not have UPC data, provide it as "Does Not Apply" value by choosing a corresponding option from the drop-down menu.

You may also indicate a UPC value for each variation of Configurable, Grouped, Bundle, Downloadable, and Simple with Custom Options Products via the Manage Variations pop-up. For that, navigate to eBay Integration > Listings > M2E Pro and open the necessary Listing. Click Manage Variations next to the Variational Product.

In the opened pop-up, click Set in the eBay Catalog Identifiers column. Enter a valid UPC value for your product variation and confirm the changes:

For each variation of your Bundle, Downloadable and Simple with Custom Options Products, you can provide Catalog ID values only via the Manage Variations pop-up.

Note: If UPC values are specified in eBay Global Settings, the relevant values will be taken from Magento Child Products. Values provided via the Manage Variations pop-up will have a priority over the ones specified in Global Settings.