(u_u)

ROS melodic:GPG error: http://packages.ros.org/ros/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F42ED6FBAB17C654

apt update fails / cannot install pkgs: key not working? - ROS Answers: Open Source Q&A Forum

keyがbuild.ros.orgの最近のセキュリティインシデントで変更されたようです。

on Ubuntu 16.04 and newer (with ROS Kinetic, Melodic, etc):
$ sudo apt-key del 421C365BD9FF1F717815A3895523BAEEB01FA116
$ sudo -E apt-key adv --keyserver 'hkp://keyserver.ubuntu.com:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
on Ubuntu 15.10 and older (ie: 14.04, from here):
$ sudo apt-key del B01FA116
$ sudo -E apt-key adv --keyserver 'hkp://keyserver.ubuntu.com:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654