However, if you understand the risks and are familiar with apt-pinning, you can add and manage 3rd party repositories. I do not recommend you do this to your Debian install and instead install the package from the official stable or back-ports repository. If not there installing the package from source is another viable method.
Again, if you require this package and it is not found in the repos and you want to maintain it using apt, as long as you understand the risks you can follow these steps.
Issue at hand
You are attempting to add the
ppa:nextcloud-devs/client repository using the
add-apt-repository command. You are unable to get
apt-get update to work with the new repo as it uses Ubuntu version names and your
sources.list being Debian uses Debian version names.
I am referencing this guide on how to add an Ubuntu ppa to your Debian system.
You will run the following command as sudo (or root):
Now you want to check your
/sources.list.d/. For example if you are adding Launchpad to your wheezy
/sources.list.d/, it should look like this:
deb http://ppa.launchpad.net/webupd8team/java/ubuntu wheezy main
deb-src http://ppa.launchpad.net/webupd8team/java/ubuntu wheezy main
Now change wheezy to the Ubuntu version you want to use. For example, xenial or 16.04.
deb http://ppa.launchpad.net/webupd8team/java/ubuntu xenial main
deb-src http://ppa.launchpad.net/webupd8team/java/ubuntu xenial main
Now you should user apt-pinning to make sure that your system maintains some sanity. I highly recommend that you make your stable repos have a higher priority than your ppa. Your
/etc/apt/preferences.d/ should have a
reponame.preferences for each repo and the contents of each should be like this:
Pin: release a=stable
Pin: release a=your-ppa-name
Now you can run
apt update and the ppa should be successfully installed.
I recommend against doing this and would suggest that you use a distro that supports the package you wish to install and avoid using third party repos. However if you know what you are doing this should work.
If you have any questions or concerns about this post, do not hesitate to ask me. If there are any corrections or misconceptions in this answer please inform me. I can update the post as necessary.
Best of Luck!