Rank Math takes the schema data from your product pages automatically and turns it into Schema-ready content. Likewise, the aggregateRating and review fields are taken from your actual product reviews that your buyers/customers leave on your products. http://i.mythemeshop.com/cIBqH4
If you don’t have any reviews yet, that is going to show up as a warning in the schema testing tool. That’s because Google will actually look for those fields.
Please acquire some reviews and ratings to get rid of the aggregateRating and review warnings.
You can fix the SKU and availability issue easily as well:
To fix the brand warning, head over to WordPress Dashboard > Rank Math > General Settings > WooCommerce and choose from any of the available fields to use as your product’s brands.
If you want to use a custom value for your Brands, you can do that by using the following code in your theme’s functions.php file: