UnityEngine missing using directive or assembly reference, what's wrong?

Question

Unity

Unity 5 (2018.2.1f1 Personal)

Answer 1

MonoDevelop 5.9.6 removed from Unity Installers and support for it has been deprecated in Unity.
MonoDevelop 5.9.6 has been replaced by Visual Studio for Mac on macOS as the bundle C# script editor in the macOS installer. Visual Studio 2017 Community is now the only C# script editor installed on Windows.
Unity no longer recognizes MonoDevelop as the “MonoDevelop (built-in)” external script editor in preferences, when it is installed in the default location next to the Unity executable. When no C# code editor is installed and selected in preferences, Unity uses the system default application for opening C# (.cs) scripts.
so. essentially an installer from Unity - checkbox to install visual studio and work in it