$ sudo apt-get update
$ sudo apt-get install libnetfilter-conntrack3 

libnetfilter-conntrack3 is installed in your system.

Make ensure the libnetfilter-conntrack3 package were installed using the commands given below,

$ sudo dpkg-query -l | grep libnetfilter-conntrack3 *

You will get with libnetfilter-conntrack3 package name, version, architecture and description in a table

libnetfilter-conntrack3 package basic information: