You're seeing this message because Google is looking for markup on a product page so they can provide detailed information like price, availability, and review ratings in their search results. Recommended properties include a nested aggregateRating of the product, an offer to sell the product and a nested review.
Google offer more information about this in their developer docs for structured data. They explain how to provide this data and provide a structured data testing tool so you can test the code before adding it. Right now, you can only do this manually, but this is obviously not helpful. It is something you want Squarespace to create automatically. Some fields are already created (price, currency, stock level) but not the ones mentioned above.
You can provide feedback about this to Squarespace by raising a ticket through Customer Care. They'll document your feature request and share it with the appropriate team.