W: GPG error: http://archive.canonical.com lucid InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 code example
Example: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6494C6D6997C215E