$ sudo apt-get update
$ sudo apt-get install elpa-markdown-mode 

elpa-markdown-mode is installed in your system.

Make ensure the elpa-markdown-mode package were installed using the commands given below,

$ sudo dpkg-query -l | grep elpa-markdown-mode *

You will get with elpa-markdown-mode package name, version, architecture and description in a table

elpa-markdown-mode package basic information: