I have used Handbrake, quite awhile ago, to convert one of my bought DVD movies to an MP4 movie file, so that I can watch the movie from a thumb drive, plugged into my smart TV.
You also have to install on your Distro the libdvdcss files, to allow Handbrake to decode the movie dvd.
Today, I tried to use Handbrake to make an MP4 movie file, from a movie DVD, but Handbrake would not work.
The notebook computer I am using has the Mint Distro, but the version I am using is no longer supported, so maybe that may be the problem?
However, I then went to my other notebook which is running an up to date version of MXLinux, and I ran into the same problem of Handbrake not being able to decode the movie DVD.
I went and re-installed the libdvdcss file and then Handbrake worked, decoding the movie dvd and creating an mp4 movie file. :-)
Strange, that I had to re install the libdvdcss on an MXLinux, that was fully updated.
Link to libdvdcss https://www.videolan.org/developers/libdvdcss.html
I think it's a legal issue, possibly. They can provide the file online, but including it with the install can get them in hot water. I don't think any Linux distributor has gotten in trouble but I guess some are more wary than others.