Hi Ayman,
Welcome to the Apple Community.
Did you try Step 9 in the article ?
says: If you still can't enroll your face, take your device to an Apple Retail Store
or Apple Authorized Service Provider or contact Apple Support.
There is a also a "Get Support" link to Apple at top right on this discussion page.
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Options:
Visit an Apple Store,
a Genius Bar reservation is recommended:
https://www.apple.com/retail/geniusbar/
or
From iOS device use the app to make a online Genius Bar appointment:
https://itunes.apple.com/us/app/apple-support/id1130498044?mt=8
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OR
See a list of Apple phone numbers around the world.
https://support.apple.com/en-us/HT201232
If your country is not listed:
Apple Assistance:
Choose your country or region:
https://support.apple.com/choose-country-region
All the best :-)