Reading package lists... Done
W: GPG error: http://deb.fdmpkg.org bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY <key>
E: The repository 'http://deb.fdmpkg.org 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.
check the sourse list
sudo ls /etc/apt/sources.list.d
you'll see a list here
...
freedownloadmanager.list
freedownloadmanager.list.save
...
then remove the problematic ppa, here free download manager is the culprit
remove them
sudo rm -i /etc/apt/sources.list.d/freedownloadmanager.list
sudo rm -i /etc/apt/sources.list.d/freedownloadmanager.list.save
Greetings from the Free Download Manager team! Here is our latest update regarding the issue. We have created a bash script that you can use to check the presence of the malware in your system. Please review our instructions on our official page: https://www.freedownloadmanager.org/blog/?p=664
We once again sincerely apologize for any inconvenience that might have been caused.