Error 21916564: The specified listing category does not support multi-variation (multi-SKU) listings.

To see which eBay leaf categories on a specific eBay marketplace support multi-variation listings, you can use the getListingStructurePolicies method of the Metadata API. If this method is used without filters, all leaf categories on the specified marketplace are returned. A variationsSupported boolean field is returned for each category, and a value of true indicates that the category supports multi-variation listings. The filter query parameter of the getListingStructurePolicies method can also be used to see if one or more leaf categories support variations.