eBay rejects your listing and gives this error when the category you selected is not the last of it's segment on the category tree. eBay expects that, when selecting your category, you follow through the category tree selection to the very last segment where there's no other option.

To correct this select the right catgory (in the last category leaf), then save and publish your listing again to have it successfully published.

Contact support at customer-success@syncommerceapp.com if the problem persists.