How to play this file?

I had the exact same problem. This answer here solved the problem.

I followed the last one , i.e,

Install vlc with sudo apt-get install vlc and then install the required plugins to play *.mkv files by sudo apt-get install vlc-plugin-libde265

Note that, if the plugin installation gave you an error of not found, you will need to add it to the repo first

sudo apt-add-repository ppa:strukturag/libde265
sudo apt-get update

Now try opening the file in vlc.