Can’t powerdown Apple watch

With WatchOS 9 powering down Apple Watch process changed.

Press and hold the side power button as you have always done. But you won't get the slider to shut off until you press the small power icon button at the top right of the screen. Tap the power icon button, then you'll get the swipe to power off your Apple Watch.

The post above is how it worked prior to WatchOS 9.

Hello, Pametjan,

Thanks for using Apple Support Communities, it's great to have you here.

Restarting your Apple Watch should be as simple as pressing and holding the side button until you see the Power Off slider. If that isn't working - if you're not seeing the Power Off slider - then we want to recommend that you try and perform a force restart on your Apple Watch. This will force your watch to reboot at which point you can test the issue again to see if the Power Off slider will reappear. To force your Apple Watch to restart, complete these steps: "Press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo." This info can also be found here: Restart your Apple Watch - Apple Support

After your watch has restarted, see if you are able to access the Power Off slider by pressing and holding the side button. Hopefully this does the trick!

Cheers.