The following signatures couldn't be verified because the public key is not available: NO_PUBKEY <key>

no_pubkey.md

Problem

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.

Solution

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. We acknowledge the reports regarding the security concerns and assure you that we're actively investigating them. As of now, all links on the FDM website are secure and functional. For a comprehensive overview of the situation, we've made an official announcement on our website. We encourage everyone to get more insights here: https://www.freedownloadmanager.org/blog/?p=664

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.