Would it be possible to clarify which Edition/License you are using for this site? The permissions based on roles UI element that you are referring to is considered "Document level Permissions." According to the feature matrix that feature is not available in the Free Edition. I apologize for the inconvenience.
If you are using a higher edition please let us know so that we can take a closer look. We may request that you provide a backup of your project to firstname.lastname@example.org
Please let me know if I can be of further assistance.