Ebay responds with this error message when, during the update of an advertisement with variants (sent, therefore, with drop-down menu on Ebay), a value has been configured in Prestashop that does not correspond to what has already been sent on eBay. The latter in fact does not allow changing the name or value of a variant to an already existing advertisement if not closing and republishing it. EXPERT USERS: If you want to understand what value has been changed, we suggest putting the module in debug (Utility section) and synchronizing a product that releases the error. At that point, consult the request to Ebay and scroll through the list of variants sent, comparing it with the data already on eBay.