We need to identify if this is a credential issue.
Follow the steps in the developer.visa.com Getting started and test whether certificates are good.
Use the instruction and install the certificates. Follow exact steps
Use SOAPUI and test hello world with your certs. That is tell if there is a credential issue.
If that works, then send us the json http request and response (complete request and response with header values) and we can help if there is an issue at support email firstname.lastname@example.org. we will need ec-correlation ID to investigate.