Xcode 13.3: Why is the inspect preview icon grayed out in the canvas view?

I'm trying to access the 'inspect preview' panel in the canvas but its icon is grayed out.

I am not running the app in the simulator. I've cleaned the build folder. There are no errors in the code. There are no error messages in the console.

struct ContentView_Previews: PreviewProvider {

    static var previews: some View {

            ContentView()

                .previewInterfaceOrientation(.portrait)

    }

}

Why are the four rightmost icons grayed out? What do I need to do to make them clickable?