Since Unity 2019.2x release, Unity has made it impossible for us to use MonoDevelop-Unity as IDE (with autocompletion & context recognition) by removing the script runtime .NET 3.5 from the editor for all the newer releases.
Unfortunately Unity has stopped supporting Monodevelop-Unity on 2018.1.0 release. However you can still use Monodevelop-Unity by downloading the Unity Installer of version 2017.4.7 from this link: Download Archive , choosing custom install and selecting to install only Monodevelop. After it’s installed in your PC, you can select it as the script editor in any version of Unity.
MonoDevelop should always install as part of the default installation so that’s a little worrying. However, if you’ve managed to get it downloaded you can tell Unity which script editor to use by navigating to:
Edit > Preferences > External Tools