While all other iPhone, you just hold the power button on the right and then "slide to power off" to turn off your phone. But this cannot be done on the iPhone X. 

The Shut Down button appear in Settings > General since iOS 11.  Apple has changed way to turn off iPhone with the Sleep/Wake button to turn off the iPhone X when needed.

When you want to turn off the iPhone X, just open the Settings and click General. Then scroll down to the bottom and click Shut Down to open up the option to slide to power off.

To restart it, you need to  press and hold the power button on the right side for a few seconds. In addition, you can plug the Lightning cable to charge the battery, it will also turn on the iPhone X.