I've seen a lot of people having some difficulties figuring out how to get the power off/restart screen to pop up. Recently, Bixby pops up when you hold the power button, BUT don't worry! You can easily change this in settings to instead show the power/restart screen. If you enjoy that "hold for Bixby" option, you can shut down/restart your phone by going to the notif panel and scrolling all the way to show the tools. You should see a little power icon right next to the settings icon and power off/ restart your phone that way. Hope this helped.