Sorry Carolh34.
At the bottom of your 1st screenshot in the original question, I've read iPhone 6 (but obviously that product list is not up to date, and I ignored what you stated in your question = iPhone 8). My fault.
In order to perform a forced restart of an iPhone model 8, these are the correct gestures:
Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the side (power) button. Only when the Apple logo appears (it might take 10-20 seconds), release that button (ignore all other screens that might appear, the one with the "slider" to turn the device off included).
No, you don't have to click on "lists" on the upper left corner, you must click on "Iphone (tous)" appearing on your screenshot.
If nothing happens after trying a forced restart and/or clickong on iPhone (tous), as already suggested, please contact Apple Support directly:--> Contact Apple for support and service - Apple Support
Have a nice Sunday.
Giulio