While running sudo apt-get update command if you are getting the error like this:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A6A19B38D3D831EF Reading package lists... Done
The solution is to get the key for the key present after NO_PUBKEY text in the error.
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys A6A19B38D3D831EF
In the above case, they key is A6A19B38D3D831EF.
After running the above command the sudo apt-get update will run without any errors.