The error message "Cannot revise a Multi-SKU item when item level SKU is supplied" occurs on an Ebay listing stock sync when the listing-level SKU is identical to one of its variant-level SKU's. This error will cause stock sync for this listing to fail.
To resolve this, either remove or change the "Custom label (SKU)" on Ebay so that it's no longer identical to any of its variant's SKU's. Ebay considers a listing that has one or more variants as a multi-SKU item.
Edit the item in Ebay
Locate the Custom label(SKU)
Update or remove the Custom label(SKU)
So that it does not match any of its variant SKU's
Save changes in Ebay
Perform a listing sync in SKULabs
Note: SKULabs delays last failed stock push jobs by an hour. After an hours period of time check to make sure the stock sync worked as expected.