Error 21916585: Duplicate custom variation label.

One or more variations defined under the Variations container have duplicate Variation.SKU values. Please note that if a seller does not provide their own custom SKUs for these variations, eBay will create them automatically by using the pivot aspects (e.g. Color and Size). So, this error could also be triggered if you had one or more variations with duplicate Color/Size combos (e.g. two 'Blue, Large' variations).