$ sudo apt-get update
$ sudo apt-get install libapache2-mod-webauth 

libapache2-mod-webauth is installed in your system.

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

$ sudo dpkg-query -l | grep libapache2-mod-webauth *

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

libapache2-mod-webauth package basic information: