Why is the checkout page empty?

Hyvä is a “Bring your own checkout” theme. We offer three choices, but you have to choose and install the option you prefer:

• The Hyvä Checkout, a prime customizable checkout available as a separate product from Hyvä
• The React Checkout, an open source community supported checkout for developers with react experience.
• The Luma Checkout, which includes a module that allows falling back to a different theme on specific pages.
  This enables you to use the standard Magento checkout with any standard Magento checkout extension.

A list of third party checkouts can be found in the Compatibility Module Tracker.

Please see Understanding Luma based checkouts for more information about how they are integrated into Hyvä.