UE4 When You Can’t Compile C++ Source Code or Can’t Add New C++ Class inside the Editor

1. Shut down the Unreal Engine.
2. Navigate to your project folder, where there are files like Visual Studio solution file, and Unreal Engine project file.
3. Right click your Unreal Engine project file.
4. Select “Generate Visual Studio Project Files”. If there is no option like this, follow the steps that are described in this link.
5. Now double click the Unreal Engine project file, and the Unreal Engine editor will open up.
6. Now you can add new C++ source code and then compile it inside the editor.