The API response code is the response the server receives in return for the request payload. What API are you trying to use? If you refer to the API's "API Reference" page, there is a sample request payload and a sample response payload. The sample response payload shows what a 200 Success API call should look like.
Please follow the steps in the “Working with Visa APIs” link below to register and create a Visa Developer application.
This guide walks you through the key steps for application creation, credentials, and connection validation.