The 'gender' attribute must be 'male', 'female', or 'unisex'. Free-text values like 'boys' or 'mens' are rejected.
Microsoft Merchant 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.