volchok@lunchbox:~$ sudo apt update
[sudo] password for volchok:
Ign:1 http://dl.google.com/linux/chrome/deb stable InRelease
Hit:2 http://ppa.launchpad.net/overcoder/hexchat/ubuntu xenial InRelease
Hit:3 http://repository.spotify.com stable InRelease
Get:4 http://dl.google.com/linux/chrome/deb stable Release [1,189 B]
Hit:5 http://security.ubuntu.com/ubuntu xenial-security InRelease
Get:6 http://dl.google.com/linux/chrome/deb stable Release.gpg [819 B]
Hit:7 http://us.archive.ubuntu.com/ubuntu xenial InRelease
Hit:8 http://ppa.launchpad.net/videolan/stable-daily/ubuntu xenial InRelease
Err:6 http://dl.google.com/linux/chrome/deb stable Release.gpg
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6494C6D6997C215E
Hit:9 http://us.archive.ubuntu.com/ubuntu xenial-updates InRelease
Hit:10 http://us.archive.ubuntu.com/ubuntu xenial-backports InRelease
Fetched 819 B in 2s (344 B/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://dl.google.com/linux/chrome/deb stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6494C6D6997C215E
W: Failed to fetch http://dl.google.com/linux/chrome/deb/dists/stable/Release.gpg  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6494C6D6997C215E
W: Some index files failed to download. They have been ignored, or old ones used instead.
volchok@lunchbox:~$