mjhjmtu

I found the program flameshot and tried to install it on Debian Stretch but got the info

 E: Unable to locate package flameshot 

On the site there is the info

Package for Debian 9 ("Stretch") also available via stretch-backports.

Please tell me how to install it via stretch-backports?

I added stretch-backports string in /etc/apt/source.list next to each stretch main contrib non-free and got some error while update but some entries did not give the error.

s@lokal:~/Dropbox$ sudo apt update
Ign:1 http://ftp.task.gda.pl/debian stretch InRelease 
Hit:2 http://ftp.task.gda.pl/debian stretch-updates InRelease 
Hit:3 http://ftp.task.gda.pl/debian stretch Release 
Hit:4 http://security.debian.org/debian-security stretch/updates InRelease 
Hit:5 http://nginx.org/packages/debian stretch InRelease 
Ign:6 http://dl.google.com/linux/chrome/deb stable InRelease 
Hit:7 http://dl.google.com/linux/chrome/deb stable Release
Hit:8 https://packages.sury.org/php stretch InRelease
Hit:10 https://download.sublimetext.com apt/stable/ InRelease
Hit:11 https://download.sublimetext.com apt/dev/ InRelease
Reading package lists... Done 
E: Failed to fetch http://ftp.task.gda.pl/debian/dists/stretch/InRelease Unable to find expected entry 'stretch-backports/source/Sources' in Release file (Wrong sources.list entry or malformed file)
E: Failed to fetch http://security.debian.org/debian-security/dists/stretch/updates/InRelease Unable to find expected entry 'stretch-backports/source/Sources' in Release file (Wrong sources.list entry or malformed file)
E: Failed to fetch http://ftp.task.gda.pl/debian/dists/stretch-updates/InRelease Unable to find expected entry 'stretch-backports/source/Sources' in Release file (Wrong sources.list entry or malformed file)
E: Some index files failed to download. They have been ignored, or old ones used instead.
s@lokal:~/Dropbox$ 

The problem remains

s@lokal:~/Dropbox$ sudo apt-get install flameshot
Reading package lists... Done
Building dependency tree 
Reading state information... Done
E: Unable to locate package flameshot
s@lokal:~/Dropbox$ 

How to install flameshot?

It's available in stretch-backports.
https://packages.debian.org/stretch-backports/flameshot

Add the backport repository.

Update, upgrade.

It should then be installable