how do I power off my iphone 12 pro max when slider doesn't work

Try a forced restart.

• Press and quickly release the volume up button

• Press and quickly release the volume down button

• Press and hold the side button.

• When the Apple logo appears, release the button.

Force restart iPhone - Apple Support (IN)

Have you also tried to turn your phone off via settings (general>Shut down)? Have you tried a hard reset to see if that resolves the slider prob? Have you updated phone to ios 15.4?