Most commercial DVDs are encrypted and will not play without decryption software.
Ubuntu Documentation Project
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# How do I enable restricted codecs to play DVDs?

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.

# שימוש בתכנית פענוח חלופית

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.

Install libdvdnav4, libdvdread8, gstreamer1.0-plugins-bad, gstreamer1.0-plugins-ugly, and libdvd-pkg.

Open a terminal window by pressing Ctrl Alt T.

עליך להריץ את הפקודה

sudo dpkg-reconfigure libdvd-pkg

ולאשר כדי להתקין את libdvdcss2.