Sizes are required for all Apparel & Accessories products targeting France, Germany, United Kingdom, and United States.
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.