Refer: https://linuxconfig.org/unable-to-play-the-file-missing-decoder-on-ubuntu-20-04-lts-focal-fossa
Error Message when trying to play MP4:
Video mp4 dvd subpicture decoder is required to play the file but is not installed
sudo apt install -y vlc
sudo apt install ubuntu-restricted-extras
sudo apt install libdvdnav4 libdvd-pkg gstreamer1.0-plugins-bad gstreamer1.0-plugins-ugly
sudo reboot
Older notes: Don't think these fixed anything; however, you do need to install VLC standard
Refer: https://itsfoss.com/play-dvd-ubuntu-1310/
These commands don't really do the trick:
sudo add-apt-repository multiverse
sudo apt install -y libdvd-pkg
sudo dpkg-reconfigure libdvd-pkg
Important: You need to install VLC from command line and NOT Snap version
sudo apt install -y vlc