Feed payload contains invalid or missing required fields - typically a missing shipping weight, zero values, or special characters in mandatory columns. The whole row is rejected before product is created.
Walmart Seller Center's product schema has format and value rules per attribute. This issue is typically: missing a required attribute for the category, an invalid enum value (e.g., 'XL' submitted where 'M, L, XL' is the valid set), out-of-range numbers, or attributes that conflict across the feed.