Install libgtk-dotnet3.0-cil by entering the following commands in the terminal:
sudo apt update sudo apt install libgtk-dotnet3.0-cil
Description:
GTK.NET library
This package provides the gtk-dotnet assembly which contains .NET extensions for GTK+. . GTK# 3.0 is a CLI (.NET) language binding for the GTK+ 3.0 toolkit and related libraries.
Homepage: http://www.mono-project.com/GtkSharp
Version: 2.99.3-2
Section: universe/cli-mono