Error 21916664: Variation Specifics provided does not match with the variation specifics of the variations on the item.

The same set of variation specific names (aka pivoting aspects) must be used for each variation in the listing. For example, you can't use just 'Color' and 'Size' for some variations, and then use 'Color', 'Size', and 'Fabric' for other variations. Variation names are set in the Variation.VariationSpecifics.NameValueList.Name field.