Spec template's hidden header rows (1-6) were modified or stripped, so Walmart can't map columns to attribute metadata. Always download a fresh spec.
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.