Either no GTIN was submitted or the value submitted is wrong; products need either a correct GTIN or both MPN and brand.
Product identifiers (GTIN, MPN, brand) anchor Google's matching of your product to its catalog. Issues here typically come from non-numeric characters in GTIN, mismatched UPC vs EAN length, brand-GTIN ownership conflicts in GS1's database, or duplicate GTINs across multiple SKUs.