Thanks! Soon after installing Ubuntu, I had disabled the updates (by setting it to use only security updates, see attachment picture).

I did not know that changing that would change my system sources.

I changed it back to `All updates` and now I was able to install `libglib2.0-dev`
```
$ sudo apt-get install nemo
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  cinnamon-desktop-data cinnamon-l10n fonts-lato gist hwdata javascript-common libcinnamon-desktop4 libgail-3-0 libjs-jquery libnemo-extension1 libruby2.7 libxapp1 nemo-data
  nemo-fileroller rake ruby ruby-json ruby-minitest ruby-net-telnet ruby-power-assert ruby-test-unit ruby-xmlrpc ruby2.7 rubygems-integration xapps-common
Suggested packages:
  apache2 | lighttpd | httpd ri ruby-dev bundler
The following NEW packages will be installed:
  cinnamon-desktop-data cinnamon-l10n fonts-lato gist hwdata javascript-common libcinnamon-desktop4 libgail-3-0 libjs-jquery libnemo-extension1 libruby2.7 libxapp1 nemo nemo-data
  nemo-fileroller rake ruby ruby-json ruby-minitest ruby-net-telnet ruby-power-assert ruby-test-unit ruby-xmlrpc ruby2.7 rubygems-integration xapps-common
0 upgraded, 26 newly installed, 0 to remove and 118 not upgraded.
Need to get 11,3 MB of archives.
After this operation, 61,0 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://br.archive.ubuntu.com/ubuntu focal/main amd64 fonts-lato all 2.0-2 [2.698 kB]
...
```

First I had tried to downgrade it, but it was about to uninstall everything on my computer:
```
$ sudo add-apt-repository universe
'universe' distribution component is already enabled for all sources.

$ sudo add-apt-repository multiverse
'multiverse' distribution component is already enabled for all sources.

$ sudo add-apt-repository restricted
'restricted' distribution component is already enabled for all sources.

$ sudo apt install --reinstall libglib2.0-0/focal
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Selected version '2.64.2-1~fakesync1' (Ubuntu:20.04/focal [amd64]) for 'libglib2.0-0'
The following packages were automatically installed and are no longer required:
  accountsservice-ubuntu-schemas apturl-common fprintd gir1.2-accountsservice-1.0 gir1.2-gck-1 gir1.2-gcr-3 gir1.2-gdesktopenums-3.0 gir1.2-gdm-1.0 gir1.2-geoclue-2.0
  gir1.2-gnomebluetooth-1.0 gir1.2-gnomedesktop-3.0 gir1.2-goa-1.0 gir1.2-graphene-1.0 gir1.2-gweather-3.0 gir1.2-json-1.0 gir1.2-mutter-6 gir1.2-nm-1.0 gir1.2-nma-1.0 gir1.2-rsvg-2.0
  gir1.2-upowerglib-1.0 gjs gnome-screensaver gnome-session-bin gnome-session-common gnome-shell-common gnome-startup-applications indicator-applet indicator-appmenu indicator-bluetooth
  indicator-datetime indicator-keyboard indicator-messages indicator-power indicator-printers indicator-session indicator-sound jayatana libbamf3-2 libcue2 libfcitx-config4
  libfcitx-gclient1 libfcitx-utils0 libfprint-2-2 libgjs0g libgnome-autoar-0-0 libgnome-panel0 libgsf-1-114 libgsf-1-common libido3-0.1-0 liblightdm-gobject-1-0 libmozjs-68-0
  libpam-fprintd libtracker-control-2.0-0 libtracker-miner-2.0-0 libunity-gtk2-parser0 libunity-gtk3-parser0 libunity-settings-daemon1 liburl-dispatcher1 nautilus-data python3-dateutil
  python3-software-properties switcheroo-control ubuntu-touch-sounds unattended-upgrades unity-greeter unity-gtk-module-common unity-gtk2-module unity-gtk3-module unity-settings-daemon
  unity-settings-daemon-schemas xwayland
Use 'sudo apt autoremove' to remove them.
The following additional packages will be installed:
  policykit-1-gnome
The following packages will be REMOVED:
  apturl gdm3 gnome-characters gnome-shell gnome-shell-extension-appindicator gnome-shell-extension-desktop-icons gnome-shell-extension-ubuntu-dock gstreamer1.0-packagekit gvfs-bin
  libglib2.0-bin light-locker light-locker-settings lightdm nautilus nautilus-share packagekit packagekit-tools software-properties-common software-properties-gtk tracker tracker-extract
  tracker-miner-fs ubuntu-desktop ubuntu-desktop-minimal ubuntu-session ubuntu-settings
The following NEW packages will be installed:
  policykit-1-gnome
The following packages will be DOWNGRADED:
  libglib2.0-0
0 upgraded, 1 newly installed, 1 downgraded, 26 to remove and 0 not upgraded.
Need to get 1.309 kB of archives.
After this operation, 17,2 MB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.
```