With the Terminal API when you create a device_code you are required to pass in a location_id and in the header there is an an access token that is specific to your account. Also when you sign up for Square you must verify your identity in order to process production payments. If you don’t verify your identity you can still access your Square account but you can’t take a production payment till you pass verification.
When the Terminal is correctly configured it will say “Powered by Square.” Previously the screen did say “Ready for payment” but it has since been updated. What’s your application Id so we can look into the 400 error? 