Most common GTIN related errors/warnings in Google Merchant
Invalid value [gtin]
The value for the GTIN is incorrect. Maybe there is a typo. You need to use a globally valid GTIN. 13 digits for EAN or 12 digits for UPC.
GTIN not related to brand
The GTIN you use is not registered for the brand you have set in the g:brand field in the feed. Usually vendor field in Shopify. If this products is non branded, you can remove of change the vendor in Shopify.
Unsupported coupon value [gtin]
Use a GTIN that's not a coupon code (coupon range prefixes: 98-99). Coupons are not allowed to be advertised on Google Shopping.
Unsupported value due to restricted prefix [gtin]
Use a GTIN that's not in the restricted range. GTINs starting with 02, 04, or 2 are restricted.
Limited performance due to missing value [gtin]
Add your product's globally valid GTIN to increase performance in Google Shopping.
Incorrect value [identifier exists]
This item has an internationally recognized, unique identifier. In most cases this means Google found this products to be from a known brand. In was done by the value in the g:brand field (which usually comes from the vendor field in Shopify). In somes cases it's done by detecting a know image of a branded product (from other sellers). You can either submit a valid GTIN or set the g:identifier_exists field to true. If this is not an option, you can try removing the brand from the g:brand field and setting a custom GTIN for this item.
Ambiguous value [gtin]
You probably have multiple GTINs set for the same product. Each item should have only 1 GTIN set.