the source dir of debian:
if something like:
Hit:13 http://mirrors.aliyun.com/debian stretch Release
Err:15 http://mozilla.debian.net jessie-backports Release
404 Not Found
Err:16 http://ppa.launchpad.net/fcitx-team/nightly/ubuntu eoan Release
404 Not Found
Reading package lists... Done
E: The repository 'http://mozilla.debian.net jessie-backports 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.
E: The repository 'http://ppa.launchpad.net/fcitx-team/nightly/ubuntu eoan 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.
then go to
/etc/apt/sources.list.d
and rm sth,
sudo rm debian-mozilla.list fcitx-team-ubuntu-nightly-eoan.list
sudo apt update