Error API “21916734” “Pictures of Version cannot be removed when update are limited”
You can find this error when you remove one or more image of a product variation where product has already been sold at least once.
Unfortunaletly, there is no solution : this is an eBay limitation. Only workaround possibe is to remove listing from eBay, and create a new listing with relevant image.
Please go to My eBay > Listings, and End listing you cannot update. Then, in eBay Marketplace module, launch synch again.