eBay releases this error when the custom product label sent by Fastbay does not match the one already present on eBay for the item you are trying to synchronize.
This error is released only if the object was previously sent with the “Manage By Sku” option (for example from another management software for eBay), which does not allow the change of personalized label for an existing product / synchronized.
If the eBay labels of these products match the references of your products site, you can remedy the problem by activating the “Use product reference as eBay custom label” option in the Orders tab (check the respective section of the documentation for more details on this mode).
Alternatively, it will be necessary to close and repost the advertisements using the form.