Install libappindicator-doc by entering the following commands in the terminal:
sudo apt update sudo apt install libappindicator-doc
Description:
Application Indicators
A library and indicator to take menus from applications and place them in the panel. . This package contains developer documentation.
Homepage: https://launchpad.net/libappindicator
Version: 12.10.1+18.04.20180322.1-0ubuntu1
Section: doc