Walmart requires a unique product identifier (UPC/EAN/GTIN) for each listing published on their marketplace.
For jewelry products
If you would like to sell jewelry on Walmart that does not have Product IDs (e.g., private label or handmade jewelry), you can request special authorization to sell these items.
To request a UPC Exemption, create a case for Partner Support. Mention the categories you request an exemption for (Jewelry); If your items are hand made; If you are a brand/manufacturer, retailer/reseller or both. If you have a website, include your website URL with links to 2-3 products that you are proposing to list.
For not jewelry products
Buy product identifiers for your products.
References:
Walmart guide: Selling Products Without Product Identifiers