If the error you are receiving indicates that you have a 500 – internal server error, then the easiest way to troubleshoot connectivity issues is to use the SOAPUI tool.
Our Working with Visa APIS with Visa Developer guide includes steps to access the helloworld application, which is the fastest way to test your access credentials and to create sample HTTP requests that work. In the “Working with Visa APIs” guide, please refer to the “Authentication” section (https://developer.visa.com/pages/working-with-visa-apis). You will want to get your SOAPUI connection to work.
Can you please confirm once you have successfully completed the steps in the Working with Visa APIs guide (including using SOAP UI and the Hello World sample application) to validate your credentials? If the issue persists, please provide screenshots of the errors and provide us with the complete error message, full http request and response details.