This was very useful to me, I was totally blocked on this step even after restarting multiple times and trying all the possible options. I didn't have Siri enabled, or at least it was not answering my "Hey Siri" or "Siri" call. Also when I pressed the power button from 1 sec it was not showing the Siri animation icon, but instead a menu to choose the options: Emergency call, Mobility Settings, Shutdown, and no Siri. Fortunately I spot that the yellow dot on top right was on meaning that Siri was listening. And it did! So even with Siri not showing up, the power button still works with enabling Siri and the AirPlane mode can be activated. Thanks