OperationFailure: database error: not authorized for query on database.system.namespaces
Heroku Postgres does not allow for user creation. You'll want to use the connection string provided in DATABASE_URL. Actions like creating/dropping DB's and creating users is prohibited.
I think this is more a heroku problem than a django problem. It appears to be that you're not allowed to add users on the mongodb instance; were you send a username and password to plug into the django settings?
First you should connect to your mongodb terminal without any authentication, then you try to create user or user-role which you want to do.
check this link for more information. here