Error Code:      21916635 
Severity:           Serious Error 
Short Error:      Invalid Multi-SKU item id. 
Long Error:      Invalid Multi-SKU item id supplied with variations. 

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 revise). You will need to create a new listing as a Multi-SKU item.

Export the variation, delete it and then upload it back.