GPG error: http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu groovy InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 531EE72F4C9D234C code example
Example: GPG error: http://ppa.launchpad.net/git-core/ppa/ubuntu hirsute InRelease
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <PUBKEY>