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Issue
When updating the Debian based system, it may happens that the apt-get displays an error message like:

W: GPG error: http://ppa.launchpad.net maverick Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FA9C98D5DDA4DB69
W: GPG error: http://ppa.launchpad.net maverick Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A9BF3BB4E5E17B5

This is a new feature of the apt-get system that guarantee the authenticity of servers for updating Debian.

Solution
Simply type the following commands, taking care to replace the number of the key that displayed in the error message:

gpg --keyserver pgpkeys.mit.edu --recv-key 5A9BF3BB4E5E17B5
gpg -a --export 5A9BF3BB4E5E17B5 | sudo apt-key add -

You have lost the signing key - or never got it - for launchpad.
To authenticate launchpad, as it is a trusted site, do;
Terminal code:

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com EB563F93142986CE