: 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.
-
In reality what I did to resolve the problem was install "Sofware & Updates" from App Center and then disable all elementaryPlus ppa's. – gabrielzschmitz Dec 19 '19 at 22:42