W: GPG error: http://dl.google.com testing Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY

Error occurred on below ...

W: GPG error: http://dl.google.com testing Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY XXXXXXXXXX

Resolve on below ...

gpg --keyserver keyserver.ubuntu.com --recv XXXXXXXXX

gpg --export --armor XXXXXXXXX | sudo apt-key add -

sudo apt-get update

DONE