Hi there,
Currently Feature Reports are only supported with public facing Portals as the Survey123 website and required components for the Feature Reports to work use services and APIs that are currently hosted online. If your Portal is not public facing, the Survey123 website and Feature Report services will not be able to access your data and feature services that are behind your firewall, so the reports can not be generated.
We are currently working on an On-Premises installation of the Survey123 website, Web App and API, and this will help resolve some issues with non-public facing portals and disconnected environments, however currently for Feature Reports the Portal still needs to be public facing. You can find more information about On-Premises here.
Regards,
Phil.