I started receiving this error today when running "apt-get update" on our chef server:
E: The repository 'https://packages.chef.io/repos/apt/stable focal 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.
This is on an Ubuntu 20.04 server. The apt repo was added by following the steps here under Debian/Ubuntu: Chef Software Packages