Unlock using passcode to enable facial recognition?

Take a look at this article:

If Face ID isn't working on your iPhone - Apple Support

If provided steps do not resolve your issue, bring it to a Genius Bar or the nearest Apple Authorized Service Provider.

Your device will require your passcode when you do the following:

• Turn on or restart your device
• Press the Home button or swipe up to unlock your device (you can change this)
• Update your software
• Erase your device
• View or change passcode settings
• Install iOS Configuration profiles