Thanks for helping the Visa Developer Community every time you post questions and solutions to this forum!
The 'ERROR 400 - 9124 Expected input credential was not present' error usually occurs when the credentials used are invalid. Can you please confirm if you are using valid credentials. Please double check the SSL settings and make sure the keystore has all the necessary certificates imported.
If you are using SOAP UI, please make sure to select "Authenticate pre-emptively" under "Auth (Basic)" tab. Also, please double check in the request raw tab that the "Authentication Basic" is present (please refer to picture below).
For the Visa Global ATM Locator testing, have you tried using API Explorer to start testing code? API Explorer lets you choose the Product API, Endpoint and Method to see the Request and Response. Here is the link to API Explorer - https://developer.visa.com/apiexplorer
We also have Sample Code for you to use as well. Please navigate to the link below for Sample Code.
Getting Started with Visa Developer > Sample Code For Project