My I phone X is not working Face ID,why?

Yzna ~ Does your iPhone X still have its original battery?

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

Settings > Battery > Battery Health & Charging

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81% is not healthy and is to be expected after years of use and charging cycles. Have the weak, aging battery replaced and Face ID will probably then work:

iPhone Battery & Power Repair

About genuine iPhone batteries

Repair Provider Verification

(Articles found by searching Apple's Knowledge Base.)

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