apt-get: Unmet dependencies for almost every package

Question

I'm running Elementary os 0.4.1 loki. After doing some magic with apt-get I managed to broke it so some packages won't install. Then I tried to fix it with some solutions I found on Google like apt-get clean && apt-get autoremove, apt-get install --fix-missing, dpkg --configure -a, etc.. Now almost every package says 'unmet dependencies'

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 wine : Depends: wine1.6 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

...

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 pinta : Depends: libcairo2 (>= 1.14.0) but it is not going to be installed
         Depends: libglib2.0-cil (>= 2.12.10-1ubuntu1) but it is not going to be installed
         Depends: libgtk2.0-cil (>= 2.12.10-1ubuntu1) but it is not going to be installed
         Depends: libmono-addins-gui0.2-cil (>= 1.0) but it is not going to be installed
         Depends: libmono-addins0.2-cil (>= 1.0) but it is not going to be installed
         Depends: libmono-cairo4.0-cil (>= 4.0.4.1) but it is not going to be installed
         Depends: gnome-icon-theme but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

And apt-get update gives me this error at the end:

Err:20 http://ppa.launchpad.net/n-muench/programs-ppa/ubuntu xenial Release
  404  Not Found [IP: 91.189.95.83 80]
Reading package lists... Done                       
E: The repository 'http://ppa.launchpad.net/n-muench/programs-ppa/ubuntu xenial 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.

And some programs won't start now

$ gimp
Gtk-Message: Failed to load module "pantheon-filechooser-module"

How do I fix it? Should I just reinstall the system?

Answer 1

The "... does not have a Release file" part is quite common, looks like you have a ppa enabled / added that doesn't support Ubuntu Xenial, where elementary OS is build on. Please try to be careful when adding third party ppa's. Please try using newer packaging solutions like Snaps, Flatpacks or AppImages.

Looking at it's url http://ppa.launchpad.net/n-muench/programs-ppa/ubuntu this is the ppa: ppa:n-muench/programs-ppa.

If you can still install applications I would recommend using ppa-purge, to remove the ppa. It does extra things, like also uninstalling and downgrading packages coming from a ppa. (It will tell you what it will do, before yo confirm that's what you want)

sudo apt install ppa-purge
sudo ppa-purge ppa:n-muench/programs-ppa

If you can't install that package I would remove the ppa like:

sudo add-apt-repository --remove ppa:elementary-os/daily

I know, an add command, with a remove argument, it's bad, but works.