# add-apt-repository ppa:certbot/certbot
This is the PPA for packages prepared by Debian Let's Encrypt Team and backported for Ubuntu(s).
More info: https://launchpad.net/~certbot/+archive/ubuntu/certbot
Press [ENTER] to continue or ctrl-c to cancel adding it
# apt-get update
W: GPG error: http://ppa.launchpad.net/ondrej/php/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4F4EA0AAE5267A6C
W: The repository 'http://ppa.launchpad.net/ondrej/php/ubuntu xenial InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
Troubleshoot error GPG key ketika melakukan upate repo
# apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 4F4EA0AAE5267A6C
# apt-get install python-certbot-nginx