Face ID ID and passcode

Is your Face ID & Passcode functioning? You are trying to change your passcode, is that correct? Have you tried power cycling your iPhone, or doing a force restart? Force restart iPhone - Apple Support

My mom had the same problem. Simply rebooting the phone solved the problem. To restart an iPhone without the home button, simply press the volume up, then volume down buttons, then press and hold the power button, located on the upper right side of the phone, until you see the Apple logo. I hope this helps