E: unable to locate package ros-melodic-desktop-full

edit

I tried installing ROS-melodic on my new virtual machine ubuntu18.04 using

sudo apt update
sudo apt install ros-melodic-desktop-full

and got the error: E: unable to locate package ros-melodic-desktop-full

I tried the solution suggested here and got:

Err:4 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
Reading package lists... Done
W: 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
E: The repository 'http://packages.ros.org/ros/ubuntu bionic InRelease' is not signed. 
N: Updating from such a repository can't be done securely, and is therefore disabled by default. 
N: See apt-secure(8) manpage for repository creation and user configuration details.

what's now?