Why can’t I turn my phone off when pressing the home button ?

The Home Button on its own is not used to turn off the iPhone. It never has been.

You can tap on the Sleep button to turn the screen off.

You can press and hold down the sleep button to bring up the slider to turn off the Phone. Then slide it to actually turn it off.

You can press and hold both the sleep button and the home button to restart the iPhone.

click here ➜ Turn iPhone on or off - Apple Support

The Home Button on your iPhone is primarily used for returning to the Home Screen and the App Switcher, but it has never been used for turning off the iPhone. That is the Power Button's job, which is located on the right of your iPhone.

Uzziarnold wrote:

I can’t shut my phone down when pressing the home button

Because the home button is not a power button...