apt-get update
Ign:1 http://dl.google.com/linux/chrome/deb stable InRelease
Get:2 http://dl.google.com/linux/chrome/deb stable Release [1.189 B]                                    
Get:3 http://dl.google.com/linux/chrome/deb stable Release.gpg [819 B]                                                                   
Err:3 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:4 http://mirrors.us.kernel.org/ubuntu xenial InRelease                                                                           
Hit:5 http://ppa.launchpad.net/danielrichter2007/grub-customizer/ubuntu xenial InRelease
Hit:6 http://mirrors.us.kernel.org/ubuntu xenial-updates InRelease       
Hit:7 http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu xenial InRelease
Hit:8 http://mirrors.us.kernel.org/ubuntu xenial-security InRelease      
Hit:9 http://ppa.launchpad.net/ravefinity-project/ppa/ubuntu xenial InRelease
Hit:10 http://mirrors.us.kernel.org/ubuntu xenial-backports InRelease    
Hit:11 http://ppa.launchpad.net/webupd8team/java/ubuntu xenial InRelease
Hit:12 http://ppa.launchpad.net/wfg/0ad/ubuntu xenial InRelease
Hit:13 https://deb.opera.com/opera-stable stable InRelease
Fetched 819 B in 1s (547 B/s) 
Reading package lists... Done
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.