I have created one site as newsite and user as Nitin who will be the author for my site so, that he can only have access rights for this site, but once I m selecting below permission I m not able to log in on AEM dashboard.
In order to user to have access to dashboards, user must have read access to root heirarchy. the user then be able to access sites and assets. You can then especically remove access to /home or /system if required but for dashboards to works, read access is required.
I would also suggest to create project specific author group and allocate permissions to that group first. You can then add users to this group. This makes user management easy.