Walmart's offer pipeline can't find an item record for the SKU you're updating. The item-side feed either failed, hasn't finished processing, or the SKU is mistyped on the offer feed.
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.