Ebay: How to Solve eBay Error Code 21916735

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.