You are trying to access the below URL REST API in Postman:
http://<servername:port>/ppm/rest/v1
and it does not work.
However you are able to open the below link
http://<servername:port>/niku/rest/describe/index.html
Here is what you see:
Release : Any
The reason it doesn't work is that it shows 401 Unauthorized.
/ppm/rest/v1/projects
The list of other requests you can use is available at: Clarity REST Documentation