: 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
One valid way to get those icons is to clone the GitHub repository elementaryPlus and use the install.sh script to install it.
sudo add-apt-repository --remove ppa:cybre/elementaryplus to remove the obsolete repository.