Common issue
The barcode looks right and is still rejected.
A GTIN is not just a number. It has a fixed set of lengths and a check digit that has to agree with the digits in front of it.
What makes a GTIN invalid
Wrong length. Valid GTINs are 8, 12, 13 or 14 digits. A value with any other length cannot be a GTIN.
Not digits. Spaces and dashes are tolerable, letters are not.
A coupon prefix. Some prefixes are reserved and are not product identifiers at all.
A failing check digit. The final digit is calculated from the others. If it does not agree, the value is either mistyped or invented.
When the product genuinely has no GTIN
Handmade, custom, own-brand and one-off products often have no manufacturer identifier. That is a legitimate case, and it is declared rather than faked: the product record should state that the identifier does not exist, and the barcode field should stay empty.
A product declared as having an identifier while the barcode is empty is the contradiction worth fixing.