root@lubuntu:/etc/apt# apt update
Hit:1 http://archive.ubuntu.com/ubuntu mantic InRelease
Hit:2 http://archive.ubuntu.com/ubuntu mantic-updates InRelease
Ign:3 https://ppa.launchpadcontent.net/lubuntu-dev/backports/ubuntu mantic InRelease
Hit:4 http://archive.ubuntu.com/ubuntu mantic-backports InRelease
Err:5 https://ppa.launchpadcontent.net/lubuntu-dev/backports/ubuntu mantic Release
  404  Not Found [IP: 185.125.190.80 443]
Hit:6 http://archive.ubuntu.com/ubuntu mantic-proposed InRelease
Hit:7 http://security.ubuntu.com/ubuntu mantic-security InRelease
Reading package lists... Done
E: The repository 'https://ppa.launchpadcontent.net/lubuntu-dev/backports/ubuntu mantic 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.