To fix the following DVD error: "This may be because the DVD is encrypted and a DVD decryption library is not installed.", you need to install libdvdread4 to play your DVD as it is likely encrypted.
Open the Terminal and on the command line, copy and paste the following commands to run them:
sudo apt-get install libdvdread4
sudo bash install-css.sh
This will allow playback of commercial DVDs that are encrypted with CSS (Content Scrambling System) and your disc should now be able to play as intended.
Also, please use VLC Media Player https://www.videolan.org/
Enjoy your movie.