The SellerZen fields require items that are created in QuickBooks Online. If you're mapping items on the Line Item page, you'll need to create the proper Service item in QuickBooks Online.
Learn more about creating and mapping the fields under the Line Item page.
Item names may be case sensitive, including spaces.
Confirm that the item exists in QuickBooks Online, and then confirm that the spelling is correct. Make sure you account for any character, including spaces.