I can't shut down my iPhone 11

Force restart it and then try to power down if that’s what you want to do.

Quickly press and release the volume up button, quickly press and release the volume down button, then hold the side button until the Apple logo appears on the screen.

Try this:

Press and quickly release volume up button

Then press and quickly release volume down button

Then press and keep holding the power button (keep holding past the swipe to power off) until the Apple Logo appears (can take 10 or more seconds, so keep holding) then let go