Walmart matched your SKU to an existing catalog page with different size/quantity. Resubmit as a variant or with the correct GTIN for your specific pack.
Walmart Seller Center uses identifiers (GTIN, MPN, brand, SKU) to match catalog records. Issues come from invalid format (non-numeric in a GTIN field), brand-GTIN mismatches in GS1's database, duplicate IDs across SKUs, or missing required identifiers for the category.