Getting the same " The repository ‘Debian Jenkins Packages binary/ Release’ is not signed." on a fresh install of Ubuntu 20.04 after running the commands specified in this post:
~$ curl -fsSL https://pkg.jenkins.io/debian-stable/jenkins.io-2023.key | sudo tee /usr/share/keyrings/jenkins-keyring.asc > /dev/null
~$ echo deb [signed-by=/usr/share/keyrings/jenkins-keyring.asc] https://pkg.jenkins.io/debian-stable binary/ | sudo tee /etc/apt/sources.list.d/jenkins.list > /dev/null
~$ sudo apt update
Ign:1 https://pkg.jenkins.io/debian-stable binary/ InRelease
Hit:2 https://pkg.jenkins.io/debian-stable binary/ Release
Get:3 https://pkg.jenkins.io/debian-stable binary/ Release.gpg [833 B]
Ign:3 https://pkg.jenkins.io/debian-stable binary/ Release.gpg
...
Reading package lists... Done
W: GPG error: https://pkg.jenkins.io/debian-stable binary/ Release: Unknown error executing apt-key
E: The repository 'https://pkg.jenkins.io/debian-stable binary/ 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 worked for me on a different system about 2 weeks ago so not sure if something changed