I assume you are using PI Web API. The PI Web API is a RESTful interface to the PI system. It gives client applications read and write access to their AF and PI data over HTTPS.
The HTTPS encryption is mandatory and do not think there is a way around this.
You can investigate the issue by enabling Debug logs (In the Windows Event Log choose PI Web API & View > Show Analytic and Debug Logs) to capture the request-response behavior on your PI Web API server.
The error code in POSTMAN response can also help you to check why your request does not seem to work.
The above steps with the debug logs and PostMan error codes should suffice in troubleshooting the issue but if you wish to capture HTTPs traffic, you can use Fiddler to decrypt HTTPs traffic, but I strongly recommend its use only in a test environment.
This is very helpful, thanks!