Brand attribute is empty, contains invalid characters, or doesn't match a known brand record. Brand is required on every Walmart listing.
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.