Install indicator-messages by entering the following commands in the terminal:
sudo apt update sudo apt install indicator-messages
Description:
indicator that collects messages that need a response
A place on the user's desktop that collects messages that need a response. This menu provides a condensed and collected view of all of those messages for quick access, but without making them annoying in times that you want to ignore them.
Homepage: https://launchpad.net/indicator-messages
Version: 13.10.1+17.04.20170120-0ubuntu2
Section: universe/gnome