Error:
GPG error: http://ppa.launchpad.net trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8BAF9A6F
Execute the following commands in terminal
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <PUBKEY>
where is your missing public key for repository, e.g. 8BAF9A6F.
Then update