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Error details:
: The repository 'http://ppa.launchpad.net/cybre/elementaryplus/ubuntu bionic Release' does not have a Release file.
W: Updating from such a repository can't be done securely, and is therefore disabled by default.
W: See apt-secure(8) manpage for repository creation and user configuration details
That repository was not updated for a very long time. As the error says, there are no packages available for Ubuntu bionic (elementary OS 5.1 is built on it) in that repository:
One valid way to get those icons is to clone the GitHub repository elementaryPlus and use the install.sh script to install it.
Use sudo add-apt-repository --remove ppa:cybre/elementaryplus to remove the obsolete repository.
