E: The repository 'http://pkg.jenkins.io/debian-stable binary/ Release' is not signed.

This error message indicates that the repository for Jenkins, located at http://pkg.jenkins.io/debian-stable binary/, does not have a valid GPG signature. This means that the package manager on your system cannot verify the authenticity of the packages being downloaded from this repository.

To solve this issue, you can try the following steps:

1. Check if the repository is still active and accessible. Sometimes, repositories may go offline or become inaccessible due to various reasons.

2. Verify that you have added the correct repository URL to your package manager's sources.list file.

3. Update your GPG keyring by running the following command in a terminal:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys KEY-ID

Replace KEY-ID with the GPG key ID of the repository. You can find the key ID on the Jenkins website.

4. Alternatively, you can also try disabling the GPG signature check for this repository. However, this is not recommended as it can compromise the security of your system.

To disable signature checks, add the following line to your package manager's configuration file:

Acquire::AllowInsecureRepositories "true";

Again, this is not recommended and should only be used as a last resort.