The UPC/GTIN you're trying to use is already attached to another SKU on your account. You need to submit a SKU Override (MERGE_DUPLICATE_SKU) request.
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.