16.04 - The following signatures couldn't be verified because the public key is not available: NO_PUBKEY

When I run apt-get update, I get this error message:

Err:18 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

I understand what causes this issue. I wonder how to fix it.

EDIT:

I tried the most upvoted solution provided in the duplicate link:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6494C6D6997C215E

I got this new issue which does not seem to be resolved on this website according to the posts I saw:

Executing: /tmp/tmp.jyolnTyZuJ/gpg.1.sh --keyserver
keyserver.ubuntu.com
--recv-keys
6494C6D6997C215E
gpg: requesting key 997C215E from hkp server keyserver.ubuntu.com
gpg: keyserver timed out
gpg: keyserver receive failed: keyserver error

I want to share what I found here (running over port 80):

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 6494C6D6997C215E

And that was successful:

Executing: /tmp/tmp.BX24beQ8ze/gpg.1.sh --keyserver
hkp://keyserver.ubuntu.com:80
--recv-keys
6494C6D6997C215E
gpg: requesting key 997C215E from hkp server keyserver.ubuntu.com
gpg: key D38B4796: "Google Inc. (Linux Packages Signing Authority) " 3 new signatures
gpg: key D38B4796: "Google Inc. (Linux Packages Signing Authority) " 1 new subkey
gpg: Total number processed: 1
gpg:            new subkeys: 1
gpg:         new signatures: 3

I then run sudo apt-get update and everything went fine.