Error:

What does this mean?
Unable to list item.
UPC, ISBN or GTN can't be blank.

Reason:

Walmart requires a UPC on every item that is to be listed.

Resolution:

Use SellerChamp's Refetch Product Data feature.

In case if you've copied this item from another marketplace or it the item has been imported from another marketplace and if it does not have a UPC to it, you can try to Refetch Product Data.

SellerChamp will easily be able to locate the UPC and add it to the item.
If in case it does not, then you'll have to manually enter the UPC.