This error occurs when a mandatory attribute is not provided for a product in your feed.
Identify which attribute is missing to resolve this error and provide the required information in your product data. These errors will be highlighted in your Merchant Center Diagnostics. Review Google's requirements for mandatory attributes to ensure all necessary details are included.
If "Brand" is required, ensure every product in your feed has a specified brand.
Ensure that "Availability" is provided for all products, indicating whether they are "in stock" or "out of stock."