Accessing Drupal REST endpoint using Oauth 2 token
Just follow the steps given in https://dev.acquia.com/blog/decoupled-drupal-authentication-oauth-20, once you get the access token in postman you will also see tokens for the user details you have given at admin/config/people/simple_oauth/oauth2_token.  Just copy the access token from postman and add it to Access Token field of Authorization tab(New version of Postman) by selecting Type as oauth2 (Dont't use the access token from the path given above).

Also make sure in permission page under RESTful Web Services section you will see your custom api listed and permission for authenticated user was given.

I followed the same and it worked for me.