Why getting this ouput for apt update?

Question

I run sudo apt update but I get the following. There is no TMSU repository in my sources.list, but still I get http://ppa.launchpad.net/tmsu/daily/ubuntu artful Release' does not have a Release file. There is also some problem in accessing Stretch's backports

Get:63 http://ftp.fr.debian.org/debian unstable/main Translation-en [5,789 kB]                                                                       
Reading package lists... Done                                                                                                                        
E: The repository 'http://ppa.launchpad.net/tmsu/daily/ubuntu artful 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: Could not open file /var/lib/apt/lists/http.debian.net_debian_dists_stretch-backports_main_binary-amd64_Packages.diff_Index - open (2: No such file or directory)
E: Could not open file /var/lib/apt/lists/http.debian.net_debian_dists_stretch-backports_main_binary-i386_Packages.diff_Index - open (2: No such file or directory)
E: The repository 'http://httpredir.debian.org/debian stretch Release' is no longer signed.
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.

My /etc/apt/sources.list

# 

deb http://ftp.fi.debian.org/debian/ stretch main
# deb http://ftp.us.debian.org/debian/ stretch main
deb-src http://ftp.fi.debian.org/debian/ stretch main
# deb-src http://ftp.us.debian.org/debian/ stretch main

deb http://security.debian.org/ stretch/updates main contrib
deb-src http://security.debian.org/ stretch/updates main contrib

# stretch-updates, previously known as 'volatile'
deb http://ftp.fi.debian.org/debian/ stretch-updates main contrib
# deb http://ftp.us.debian.org/debian/ stretch-updates main contrib
deb-src http://ftp.fi.debian.org/debian/ stretch-updates main contrib
# deb-src http://ftp.us.debian.org/debian/ stretch-updates main contrib

# stretch backports
deb http://http.debian.net/debian stretch-backports main contrib non-free

deb http://httpredir.debian.org/debian/ stretch main contrib

# NO package
# deb http://cran.wu.ac.at/bin/linux/debian stretch-cran3/

# https://unix.stackexchange.com/a/366085/16920
deb http://ftp.fr.debian.org/debian unstable main

# https://cran.r-project.org/
# http://stackoverflow.com/q/44026585/54964
deb https://ftp.eenet.ee/pub/cran/bin/linux/debian stretch-cran34/
deb [arch=amd64] https://download.docker.com/linux/debian stretch stable
# deb-src [arch=amd64] https://download.docker.com/linux/debian stretch stable

Testing sgargel's proposal

masi@masi:/etc/apt/sources.list.d$ grep -R tmsu
tmsu-ubuntu-daily-artful.list:deb http://ppa.launchpad.net/tmsu/daily/ubuntu artful main
tmsu-ubuntu-daily-artful.list:# deb-src http://ppa.launchpad.net/tmsu/daily/ubuntu artful main

OS: Debian 9

Solution

The ppa.launchpad.net/tmsu/daily/ubuntu should work only on Ubuntu derivative's , From the output of grep -R tmsu (@sagargel comment's) , the ppa still exist under /etc/apt/sources.list.d , you should remove it manually :

sudo rm -i /etc/apt/sources.list.d/tmsu-ubuntu-daily-artful.list
sudo apt update

Comments

[sgargel]

Anything in /etc/apt/source.list.d/ ? I suggest you a grep -R tmsu

[Léo Léopold Hertz 준영]

@sgargel Please, see the body. What do you think?

[sgargel]

The repository is there. If you just don't need it, remove that file.