Variant Group ID, Variant Attribute Name, and 'Is Primary Variant' must all be populated for variant items, including the parent.
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.