How to reset Face ID on an unlocked iPhone 12 Pro Max?

See especially item 8 here.

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

Of course, we don’t know what you tried, what you saw, error messages, etc.

"How to reset Face ID on an unlocked iPhone 12 Pro Max?: I have an iPhone 12 Pro Max and I own it. There aren’t any company locks nor system locks. However, I am having a hard time trying to reset my Face ID on my device. Because of this, I am unable to complete certain tasks and utilize my phone properly. What should I do?"

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Face ID Issues:

A. Re-Enable FaceTime:

To reset FaceTime on your iPhone...

1. Go to: Settings
2. Tap: FaceTime
3. Turn Off: FaceTime
4. Turn On: FaceTime

B. Delete the Old FaceTime Passcode:

1. Go to: Settings
2. Tap: Face ID & Passcode
3. Enter: your current passcode when prompted.
4. Scroll Down and Tap: Turn Passcode Off
5. Tap: Confirm

C. Links to Go By:

• If an Alert Says Face ID has been Disabled on your iPhone or iPad Pro - Apple Support
• If you can’t Turn On or Sign In to iMessage or FaceTime - Apple Support
• If Face ID isn't Working on your iPhone or iPad Pro - Apple Support

Can you define “a hard time”? Is Face ID not working?

Open Settings, tap Face ID & Passcode, enter your passcode. Do you have Face ID enabled? Can you Reset it and set it up again? If not, what is the message?

Start here: When Face ID is not working on your iPhon… - Apple Community

If you have Stolen Device Protection enabled and your Face ID isn’t working you must reset your iPhone to factory settings, then restore a back.