[Error fix] Solved 'The repository 'https://packagecloud.io/shiftkey/desktop/any any InRelease' is not signed.': The following signatures couldn't be verified because the public key is not available

# Problem

when I was entered the command, I got a that Error(E):

$ sudo apt update
Hit:1 http://packages.microsoft.com/repos/code stable InRelease
...
Get:10 https://packagecloud.io/shiftkey/desktop/any any InRelease [24.4 kB]
Hit:12 http://packages.ros.org/ros/ubuntu focal InRelease  
Err:10 https://packagecloud.io/shiftkey/desktop/any any InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B7E65990E879472F
Hit:13 http://packages.ros.org/ros2/ubuntu focal InRelease
Reading package lists... Done
W: GPG error: https://packagecloud.io/shiftkey/desktop/any any InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B7E65990E879472F
E: The repository 'https://packagecloud.io/shiftkey/desktop/any any 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

Remove that packagecloud entry from either your /etc/apt/sources.list file, or one of the files in /etc/apt/sources.list.d/ , then run apt update again.

$ ls /etc/apt/sources.list.d/
google-chrome.list                                         packagecloud-shiftkey-desktop.list.save
google-chrome.list.save                                    ros2.list
graphics-drivers-ubuntu-ppa-focal.list                     ros2.list.save
graphics-drivers-ubuntu-ppa-focal.list.save                ros-latest.list
kdenlive-ubuntu-kdenlive-stable-focal.list                 ros-latest.list.save
maarten-baert-ubuntu-simplescreenrecorder-focal.list       vscode.list
maarten-baert-ubuntu-simplescreenrecorder-focal.list.save  vscode.list.save
packagecloud-shiftkey-desktop.list

$ sudo rm -i /etc/apt/sources.list.d/packagecloud-shiftkey-desktop.list
rm: remove regular file '/etc/apt/sources.list.d/packagecloud-shiftkey-desktop.list'? yes

Then, Everything is under control.

$ sudo apt update
Hit:1 http://packages.microsoft.com/repos/code stable InRelease
...
Fetched 222 kB in 4s (59.9 kB/s)    
Reading package lists... Done
Building dependency tree       
Reading state information... Done

# Reference

[Blog] sudo apt update - packagecloud.io unauthorized: https://forums.raspberrypi.com/viewtopic.php?t=311367