Could you show me a screenshot of your postman call? The first call should be to access_token, as you need to obtain that and save it before you can use other calls.
I assume you’ve added new application in CMS and the client id and secret are saved in Postman environment?
Perhaps you could have a read through this - 1.8 API Introduction
Should help you get started, if you’d still have issues, please do let me know.