20.04: Can't install gtk-recordmydesktop

It is because of it is obsoleted and currently not supported. You could install it by enabling some older repository (deb http://cz.archive.ubuntu.com/ubuntu eoan main universe for example) which contain it or download its packages from there. Enabling other ubuntu version repository should be immediately disabled just after installing. But it is a bad way which could break your system. Better use newer supported software.

Vokoscreen records a desktop, a window and an area:

sudo apt install vokoscreen

Or use vokoscreenNG:

sudo apt install vokoscreen-ng

Ignore this if you got gtk-recordmydesktop to work. I only add an alternative in case someone of you could not.

A solution that works well for me is the light-weight simple screen recorder. It is in the Ubuntu-repositories and can be installed the usual way,

sudo apt update
sudo apt install simplescreenrecorder

There is also a 32 bit OpenGL version that works on 64 bit systems. More information for this screen recorder can be found here: https://www.maartenbaert.be/simplescreenrecorder/#download

It is installable by using packages from Ubuntu 18.04 LTS:

cd ~/Downloads
wget http://archive.ubuntu.com/ubuntu/pool/universe/p/pygtk/python-gtk2_2.24.0-5.1ubuntu2_amd64.deb
wget http://archive.ubuntu.com/ubuntu/pool/universe/g/gtk-recordmydesktop/gtk-recordmydesktop_0.3.8-4.1ubuntu1_all.deb

sudo apt-get install ./python-gtk2_2.24.0-5.1ubuntu2_amd64.deb ./gtk-recordmydesktop_0.3.8-4.1ubuntu1_all.deb