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.