The Face ID is not working

That's generally a hardware issue. If the unit's on an OS version older than 16.4, you might be able to fix it by updating it from a computer or the Software Update function in Settings; otherwise, contact Apple Support.

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"The Face ID is not working"

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Troubleshooting Face ID:

Overview these two links:

• If an Alert Says Face ID has been Disabled on your iPhone or iPad Pro - Apple Support

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