If something goes wrong while processing the request, an appropriate HTTP error code is returned - for example
400 Bad Request or
500 Internal server error.
In addition, the response contains an
errors array, containing error messages - usually just one, but it can contain more in more complex scenarios - multiple validation errors for example.
In some cases, for example tax calculation, even when an error occurs, the response might contain additional information - for example country detected from an IP address. Please consult individual endpoint for detailed information.