I believe you might be querying the wrong IP address. Have you tried opening your instance using the same IP in your browser?

Please verify your IP address and replace it with 127.0.0.1. You can then make the API call from Postman to your Acumatica instance.

For example:

http://localhost/test/entity/auth/login

If you are using the same computer where the instance is hosted, you can simply use localhost instead of the IP address.