Ask the system:
walt@bat:~(0)$ apt-cache search libdvdcss
brasero - CD/DVD burning application for GNOME
gxine - the xine video player, GTK+/Gnome user interface
libdvdread4 - library for reading DVDs
python3-dvdvideo - Video DVD reader library
kubuntu-restricted-extras - Commonly used media codecs and fonts for Kubuntu
libdvd-pkg - DVD-Video playing library - installer
lubuntu-restricted-extras - Commonly used media codecs and fonts for Lubuntu
ubuntu-restricted-extras - Commonly used media codecs and fonts for Ubuntu
xubuntu-restricted-extras - Commonly used media codecs and fonts for Xubuntu
libdvdcss2 - library for accessing encrypted DVDs
libdvdcss-dev - library for accessing encrypted DVDs - development files
You need to sudo apt install
one or more of these packages.
Ignoring your "unable to do it (key not found?)" message due to lack of specific information.