You may receive this error message regarding a Walmart listing within Listing Mirror. This message comes DIRECTLY from Walmart, not Listing Mirror. Walmart requires that every item has either a UPC, GTIN or ISBN defined to list onto their marketplace.
If you are selling a book, you will be required to supply an ISBN. This information will be on the back cover of the book that you are selling.
If you are selling any other item that is not a book, you have the option to use either a UPC or GTIN. To find the UPC associated with the item that you are selling, please check out our educate article covering this topic.
After you find the UPC that is associated with your item, you will then need to go to the listings edit page for the Walmart listing. Under the keywords field on the child listing side of the listing edit page there will be the UPC field.
Input your UPC and save changes. This will then trigger a sync from Listing Mirror --> Walmart. Once the pending sync completes, your UPC error will be resolved.
Alternatively you can download a 'UPC Update' template from Template Generator to update your UPCs in bulk.
If you have any questions regarding this process, feel free to reach out to Listing Mirror support.