It has something to do with having options on products. I had the same error and looking through the profiler at the Doctrine info there was an error for 2 of the objects that were being persisted to the database. Specifically, it couldn't map Sylius\Bundle\AssortmentBundle\Entity\CustomizableProduct or Sylius\Bundle\AssortmentBundle\Entity\Variant\Variant.
To fix it I had to Generate Variants of the product which is on the product details page in the admin area. Without having variants to add to the cart, it couldn't choose a specific type of the product to add to the cart.
I feel like this should produce a better error, especially when running the app in dev mode.