For SFTP to work you need to use configure your host inside of ~/.ssh/config as password prompt is not supported. For more information on this topic please see the [official docs](https://restic.readthedocs.io/en/stable/030_preparing_a_new_repo.html#sftp) on the matter.
See [here](https://github.com/restic/rest-server) for how to install a rest server backend and [here](https://restic.readthedocs.io/en/latest/030_preparing_a_new_repo.html#rest-server) for further documentation.