As Per the Oracle Support Documentation
1. Login in to xmlpserver as weblogic user
2. Navigate to Administration > Security Center > Security Configuration
3. On the bottom of the page navigate to the Security Model section
4. On the drop down list select "BI Publisher Security" and provide a password for the security
5. Log out of xmlpserver and login as weblogic user on Weblogic Console and restart the BI Server
6. Once restarted then login back to xmlpserver with the username as administrator and provide the password you had provided on step 4.
7. Now navigate to Administration > Security Center, and a new link called "users" is available and on the Roles and Permissions link, "Create Role" button can be found.
N.B this solution is applicable to only Standalone BI Publisher
It is not recommended to change the Security Model to 'BI Publisher Security' when using BI Publisher within OBIEE installation.
For OBIEE installation the Security Model should be 'Fusion Middleware' security or 'BI Server Security'.
It Worked for me ..Just posting it if anyone run into same issue:)