As of February 29, 2016, eBay requires that all branded items that are listed will have product identifiers - Brand, Manufacturer Part Number (MPN), and Global Trade Item Number (GTIN) - provided. This error is returned by eBay when your listing does not have the MPN provided.
This error can as well occur for already existing listings on eBay that SynCommerce is trying to sync back. This is because old listings were successfully published without requiring to have this value provided, trying to update them however will require the MPN to be provided, else fail.
This can be fixed by simply going to the edit page of the failed eBay listing, provide the MPN of the item, then save and publish your listing or click the "Update listing" button.
Refresh the listings page in under a minute to see the change in status for that listing - active if successfully published or synced on your eBay store or otherwise.
Contact support at firstname.lastname@example.org if the problem persists.