updating from repositories can`t done by securely

when I run sudo apt-get update on ubuntu 19.04 it results as; Err:1 http://lt.archive.ubuntu.com/ubuntu disco InRelease Could not connect to lt.archive.ubuntu.com:80 (193.219.61.87). - connect (111: Connection refused) Cannot initiate the connection to lt.archive.ubuntu.com:80 (2001:778::87). - connect (101: Network is unreachable) Err:2 http://lt.archive.ubuntu.com/ubuntu disco-updates InRelease
Cannot initiate the connection to lt.archive.ubuntu.com:80 (2001:778::87). - connect (101: Network is unreachable) Get:3 http://deb.playonlinux.com trusty InRelease [2,590 B]
Hit:4 http://security.ubuntu.com/ubuntu disco-security InRelease
Ign:5 http://ppa.launchpad.net/gnome3-team/gnome3/ubuntu disco InRelease Err:6 http://ppa.launchpad.net/gnome3-team/gnome3/ubuntu disco Release 404 Not Found [IP: 91.189.95.83 80] Ign:7 http://download.opensuse.org/repositories/home:/strycore/xUbuntu_16.04 InRelease Get:8 http://download.opensuse.org/repositories/home:/strycore/xUbuntu_16.04 Release [986 B] Get:9 http://deb.playonlinux.com trusty/main amd64 Packages [564 B] Get:10 http://download.opensuse.org/repositories/home:/strycore/xUbuntu_16.04 Release.gpg [481 B] Get:11 http://deb.playonlinux.com trusty/main i386 Packages [564 B] Get:12 http://download.opensuse.org/repositories/home:/strycore/xUbuntu_16.04 Packages [831 B] Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/gnome3-team/gnome3/ubuntu disco Release' does not have a Release file. 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.

I cannot install any program based on sudo apt-get install I tried everything that I see so far but none of them worked for me. I would appreciate your help. Thank you.