From another forum member;
Dec 2, 2018 2:04 AM in response to johnnyb1258
I had the same issue when I was upgrading from iPhone 6s to iPhone XR (iOS 12.1). The menu Face ID & Passcode was missing from the Settings screen. I tried doing a hard reset on the phone but the menu is still missing. I tried a restore via iTunes (12.8, MacOS 10.13.6) on a computer and setup my phone as new (factory reset). As new, I was able to setup Face ID but, after i synced from the last backup, the menu Face ID & Passcode goes missing. Weird!
I finally got it to appear by changing Passcode Changes to Allow (in Screen Time > Content & Privacy Restrictions). If it's already set to Allow (mine was set to Allow 😕), I had to toggle it to Don't Allow and back to Allow. And now Face ID & Passcode is showing! 😮