From the 1st of November 2019, Google Merchant Center (GMC) will be making changes as to how it enforces Unique Product Identifiers (UPI) checks on your listings.
Previously if you did not have a UPI for a product that requires one such as a GTIN or an incorrect UPI your listing would be automatically disapproved until you resolved the issue.
The way GMC enforces issues with UPI’s is now changing, if your product does not have a UPI it will still be listed however it may not perform as well up against similar products that do have a correct UPI.
This doesn't mean you shouldn't have to worry...