The latest version of libdvdcss for macOS available is 1.4.2 and was released in 2018. You may need to have this installed in order to utilize Handbrake. This tutorial is actually broken up into two parts. The first part covers installing it with the Homebrew package manager. The second part will cover building the library from the source code. Either choice will work, it just depends on your preference, but both will require having Xcode Command Line Tools installed so that you can run the development tools from the terminal.
Installing with Homebrew
This is definitely the easiest of the two options and only requies running 3 commands.
xcode-select --install /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)" brew install libdvdcss
Building and compiling from source
The following commands will download the source code for the latest version of libdvdcss from the VideoLAN website, extract it, configure, and build the library. After it's done, you may need to move the compiled files from /usr/local/lib to /usr/lib.
curl -o libdvdcss-1.4.2.tar.bz2 download.videolan.org/libdvdcss/1.4.2/libdvdcss-1.4.2.tar.bz2 tar -xf libdvdcss-1.4.2.tar.bz2 xcode-select install cd libdvdcss-1.4.2 ./configure sudo make install