Install libayatana-appindicator0.1-cil-dev by entering the following commands in the terminal:
sudo apt update sudo apt install libayatana-appindicator0.1-cil-dev
Description:
Application Indicators for Mono
A library and indicator to take menus from applications and place them in the panel. . This package contains files that are needed to build applications.
Homepage: https://github.com/AyatanaIndicators/libayatana-appindicator
Version: 0.5.3-3
Section: universe/cli-mono