$ sudo apt-get update
$ sudo apt-get install icinga2-classicui 

icinga2-classicui is installed in your system.

Make ensure the icinga2-classicui package were installed using the commands given below,

$ sudo dpkg-query -l | grep icinga2-classicui *

You will get with icinga2-classicui package name, version, architecture and description in a table

icinga2-classicui package basic information: