Getting this error when trying to do sudo apt-add-repository on Ubuntu 18.04, I swear it was working the other day.
W: GPG error: http://webmin.mirror.somersettechsolutions.co.uk/repository sarge Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D97A3AE911F63C51
E: The repository 'http://webmin.mirror.somersettechsolutions.co.uk/repository sarge Release' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://download.webmin.com/download/repository sarge Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D97A3AE911F63C51
E: The repository 'http://download.webmin.com/download/repository sarge Release' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.