$ sudo apt-get update
$ sudo apt-get install libapache2-modsecurity 

libapache2-modsecurity is installed in your system.

Make ensure the libapache2-modsecurity package were installed using the commands given below,

$ sudo dpkg-query -l | grep libapache2-modsecurity *

You will get with libapache2-modsecurity package name, version, architecture and description in a table

libapache2-modsecurity package basic information: