All sellers at Walmart.com are required to provide at least one universal ID (UPC/EAN/GTIN) for products they wish to sell using Walmart's platform. But there are certain situations when a product doesn't have any of these three, for example:
1) Brand, manufacturer or publisher does not provide IDs (private label products, hand-made products)
2) Non-branded products that don't have IDs (wholesale products)
3) Customized products without IDs (products with customized element like a person's name)
There are two ways to start selling such products:
1) Have your products exempted from providing universal IDs. If your products belong to any of these three groups mentioned earlier, please notify Walmart about it and request exemption for your products. You need to explain to Walmart why you do not have UPC/EAN/GTIN and request a permission to sell it without the code. This way, you will be able to submit your products to Walmart without providing UPCs/EANs/GTINs. Keep in mind that if there is an available manufacturer issued UPC/EAN/GTIN, then it must be used.
2) The process of getting exemption can take some time and if a merchant has just few SKUs without IDs it is usually a good idea to buy them.