How to investigate the issue
Product identifiers help systems recognize what you sell, but different products and variants do not always share the same identifier rules. GTIN values are issued within the GS1 system. A mathematically correct GTIN check digit does not prove the number belongs to your exact item, so never generate arbitrary identifiers to fill missing information. Compare the original manufacturer packaging, product record and supplier data before making changes. If a product has a known GTIN, validate its digit length and checksum. Then check for duplicates in your catalog and investigate whether they represent legitimate grouped variants or accidental reuse. For products without GTINs, follow the current Google Merchant product data specification and use the required brand/MPN attributes when applicable. Avoid substituting internal SKUs for GTINs. Bulk checks are a useful first pass, but country rules, custom products and product categories still require human verification. Our tools do not communicate with GS1 or prove official ownership.
Relevant free diagnostic tools
Official reference
Google Merchant Center product data specification. Check current policies and your account for definitive eligibility details.