$ sudo apt-get update
$ sudo apt-get install docbook-xsl-ns 

docbook-xsl-ns is installed in your system.

Make ensure the docbook-xsl-ns package were installed using the commands given below,

$ sudo dpkg-query -l | grep docbook-xsl-ns *

You will get with docbook-xsl-ns package name, version, architecture and description in a table

docbook-xsl-ns package basic information: