Pega API uses Basic authentication by default. Your default Community Edition setup uses SSO authentication. 

You will need to do the following to be able to make REST API calls on Community Edition:

1. "Save As" your Community Edition Operator ID to a different record that has Use external authentication turned off on the Security tab.
2. While you're there, use Update password specify a password and save this with the new Operator ID.
3. Log out, remove the /sso part of the URL (leaving XXX.pegace.net/prweb/app/... as of Pega 8.4), and go to your modified URL
4. Log in using your username/password for the new Operator ID that doesn't use external authentication
5. Change your password if your Community Edition password policy requires it to be changed.

From your REST client, ensure you are using:

• The non-SSO URL pattern, and
• Your new Operator ID credentials as Basic Authentication headers.

Avoid making any Service Package changes as suggested on of the articles. Community Edition will require you use SSL.

Remember you will need to wake up your Community Edition instance from My Trials for REST API calls to get through.