Google announced that it will begin enforcing that Google Merchant Center merchants use Manufacturer Part Number (MPN) and accurate ones. Then in September of 2021 Google will disapprove merchants with these issues.
Google wrote the “Manufacturer Part Number (MPN) is a unique product identifier that is used to identify a specific product among all of the products produced by a manufacturer. Customers may search for a product using a specific MPN, so providing the MPN can help ensure that your product is shown in relevant situations.”
Then starting in September of 2021, the products for which you received warnings will be disapproved.
Google said you should “review your Diagnostics page to see if your products are affected, and either provide a valid MPN value or clear the current incorrect value in your product data.”
Forum discussion at Twitter.