$ sudo apt-get update
$ sudo apt-get install elpa-linum-relative 

elpa-linum-relative is installed in your system.

Make ensure the elpa-linum-relative package were installed using the commands given below,

$ sudo dpkg-query -l | grep elpa-linum-relative *

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

elpa-linum-relative package basic information: