IntelliSense: "#using" requires C++/CLI to be enabled

Your project settings are wrong. Specifically Configuration Properties, General, Common Language Runtime support.

Fall in the pit of success by starting your project by picking one of the project templates in the CLR node.

Choose Project -> Properties from the menu bar. In the Project properties window, under Configuration Properties -> General, make sure that Common Language Runtime Support is set to Common Language Runtime Support (/clr)

In VS2019 it the steps would be :

• 1/ Right click on the project

• 2/ Project

• 3/ Properties

• 4/ Configuration Properties

• 5/ Advanced

• 6/ Common Language Runtime Support change it to Common Language Runtime Support(/clr)