I know people have asked this question but the solutions seem to differ and I haven't found mine yet. I get the response as stated in the subject title. Here's the output in the terminal:
sudo apt-get update
Hit:1 http://repo.steampowered.com/steam precise InRelease
Ign:2 http://dl.google.com/linux/chrome/deb stable InRelease
Get:3 http://dl.google.com/linux/chrome/deb stable Release [1,189 B]
Get:4 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Hit:5 http://gb.archive.ubuntu.com/ubuntu xenial InRelease
Get:6 http://dl.google.com/linux/chrome/deb stable Release.gpg [819 B]
Get:7 http://gb.archive.ubuntu.com/ubuntu xenial-updates InRelease [102 kB]
Hit:8 http://repository.spotify.com stable 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
Get:9 http://gb.archive.ubuntu.com/ubuntu xenial-backports InRelease [102 kB]
Hit:10 http://ppa.launchpad.net/pipelight/stable/ubuntu xenial InRelease
Fetched 308 kB in 1s (200 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.
Thanks very much for any help.