WPSSO Core Pro and its WPSSO Schema JSON-LD Markup Pro add-on include a variety of additional product information in its Schema markup for WooCommerce products, including the product Brand, Color, Condition, EAN, GTIN-8, GTIN-12, GTIN-13, GTIN-14, ISBN, Material, MPN (aka Manufacturer Part Number), Size, and Gender.
WooCommerce itself already manages information on product Availability (aka Stock), Prices, SKU, etc., but other WooCommerce product attributes must be created to provide additional product information.
Google reads product information in the Schema Product markup provided by the WPSSO JSON add-on, and issues a warning if some property values are missing – like the product Brand, Condition, and the Product ID.
The Product ID referred to by Google is not the Schema ‘productID’ property, but a general term Google uses for a missing product identifier, like the ‘mpn’ property (aka Manufacturer Part Number) or another unique product identifier. The product SKU, unfortunately does not appear to count as a unique product identifier. :)