See Validation errors. The server could not understand the request. Indicates one of these conditions: The API cannot convert the payload data to the underlying data type. The data is not in the expected data format. A required field is not available. A simple data validation error occurred.

Hi, This may help you. https://www.paypal-community.com/t5/Sandbox-Environment/I-got-400-bad-Request/td-p/1529491# https://www.paypal-community.com/t5/Adding-and-updating-info/error-404-paying/m-p/736719#