In the latest Android versions, a long press on the power button does not launch the power menu. Instead, the default behavior is to launch Google Assistant, as shown below:



There are three ways you can launch the power menu instead. 

  1. The first option is to press the power button extra long, for at lest 3.5 seconds. 
  2. Alternatively, simultaneously press the power button and volume up.

    Both these actions will launch the power menu:


  3. You can also change the Android settings, so that a long press will launch the power menu rather than the assistant. To do this, open the Settings app, and go to System settings.
     

    Find the Gestures option, and click on that:


    Here you can click on Press & hold power button and set it to launch the power menu rather than the assistant.