root@ymodt:~# apt-get update
Get:1 http://security.ubuntu.com/ubuntu zesty-security InRelease [89.2 kB]
Hit:2 http://us.archive.ubuntu.com/ubuntu zesty InRelease
Ign:3 http://dl.google.com/linux/chrome/deb stable InRelease
Get:4 http://us.archive.ubuntu.com/ubuntu zesty-updates InRelease [89.2 kB]
Get:5 http://us.archive.ubuntu.com/ubuntu zesty-backports InRelease [89.2 kB]
Get:6 http://ppa.launchpad.net/graphics-drivers/ppa/ubuntu zesty InRelease [21.3 kB]
Get:7 http://dl.google.com/linux/chrome/deb stable Release [1,189 B]
Get:8 http://dl.google.com/linux/chrome/deb stable Release.gpg [819 B]
Err:8 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
Get:9 http://ppa.launchpad.net/graphics-drivers/ppa/ubuntu zesty/main amd64 Packages [11.5 kB]
Get:10 http://ppa.launchpad.net/graphics-drivers/ppa/ubuntu zesty/main i386 Packages [11.5 kB]
Get:11 http://ppa.launchpad.net/graphics-drivers/ppa/ubuntu zesty/main Translation-en [3,368 B]
Fetched 316 kB in 0s (409 kB/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.