The "https://download.webmin.com/download/newkey/repository stable Release is not signed" error occurs when APT cannot verify the Webmin repository, typically due to missing or outdated GPG keys. This issue is commonly resolved by running the official Webmin repository setup script, which automatically updates the GPG keys. For complete installation instructions, visit Webmin . How to fix Webmin repo: NO_PUBKEY 2D223B918916F2A2
