Error Code 21916635
You used the same variation option under the same parent. Therefore, you can’t create this variation on eBay. If they have the same values, the product won't be created. The original item was not a Multi-SKU item and you are trying to relist it as a Multi-SKU item. If the original item was not listed as a Multi-SKU item, you cannot change it to be a Multi-SKU item during relist or revision.
Solution
You will need to create a new listing as a Multi-SKU item. You have to export and relist the whole variation. You can change this from StoreAutomator by using edit(beta). Click to item and edit(beta). Then click Inventory Product Data. Choose the field or fields that caused the problem, in this example, Size and Color.