DVD support cannot be provided by default in Ubuntu due to legal and technical restrictions. Most commercial DVDs are encrypted and so require the use of decryption software in order to play them.
Use Fluendo to legally play DVDs
You can buy a commercial DVD decoder that can handle copy protection from Fluendo. It works with Linux and should be legal to use in all countries.
Use alternative decryption software
In some countries, the use of the below unlicensed decryption software is not permitted by law. Verify that you are within your rights to use it.
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Install libdvdnav4, libdvdread4, gstreamer1.0-plugins-bad, gstreamer1.0-plugins-ugly, and libdvd-pkg.
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Open a terminal window by pressing Ctrl+Alt+T.
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Run the command
$ sudo dpkg-reconfigure libdvd-pkg
and confirm in order to install libdvdcss2.
Reference
https://help.ubuntu.com/stable/ubuntu-help/video-dvd-restricted.html.en