The google_product_category field is capped at 255 characters; long category strings (deep taxonomy paths) may need to use the numeric category ID instead.
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.