How to Solve eBay 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.
You are getting this error because 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. You will need to create a new listing as a Multi-SKU item. You have to export and relist the whole variation.