Install libappindicator3-0.1-cil by entering the following commands in the terminal:
sudo apt update sudo apt install libappindicator3-0.1-cil
Description:
CLI binding for appindicator3 using GObject introspection
This package provides the appindicator3-sharp assembly that allows CLI (.NET) programs to use the Appindicator3 library.
Homepage: https://github.com/stsundermann/appindicator-sharp/
Version: 12.10.0+git20151221-3
Section: universe/cli-mono