Error Message: Input data for tag <Item.ItemID> is invalid or missing.
Please check API documentation.
Summary: This error means eBay does not have an Item ID for this SKU.
This happens when you click the "UPDATE & PUSH" in Zentail after making an edit to resolve another common error - likely listed on this page - that prevented eBay from creating a listing.
In this case, eBay's error API is faster than eBay's ability to update the information that Zentail has just sent.
Hang tight, your SKU will be listed within 15 minutes and this error will disappear.