Help me to do my Face ID to start working

This is generally a hardware issue. If the unit's on an OS version older than 16.5.1, 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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kety ~ Does your iPhone 11 Pro Max still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health & Charging

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