Customers on Premium and Enterprise subscription typically have access to sandbox. When customers copy data to their sandbox for testing, not all Checkout data is copied over to the Sandbox. Specially items, vendors and categories.
Because of this the above mentioned data is not copied over, Checkout will not work as intended. Therefore, the customer has to do some special things to make sure Checkout in the sandbox works. Here are required steps in this specific order.
Copy vendor data from production and import it into sandbox.
Copy category data and import it into the sandbox.
Copy item data and import into the sandbox
Connect assets to the referring items.
Once all of these steps are performed, Checkout should work as intended.