Try this then sudo add-apt-repository --remove ppa:gnome-shell-extensions/ppa followed by sudo apt-get update.
– LisoMay 08 '19 at 07:18
@Tanny The linked duplicate that Jim posted should be the same case as yours. The custom repository you are using does not support Disco (19.04), at least not yet. You can verify this here: https://launchpad.net/~gnome-shell-extensions/+archive/ubuntu/ppa. Edit into your post what you have tried exactly please, so we can help you further.
– DanMay 08 '19 at 07:42
sudo add-apt-repository --remove ppa:gnome-shell-extensions/ppa
followed bysudo apt-get update
. – Liso May 08 '19 at 07:18