Keep your eyes open and make sure your face is fully visible and not covered by any object, such as a hat or sunglasses.
To unlock your tablet, just turn your screen on and look at the front camera.
If there is a facial-recognition error, and you cannot use alternative sign-in methods to recover or reset the tablet in any way, your tablet will require service. Additional charges may apply, and all the personal data on your tablet may be deleted. For more info, contact the nearest authorised service facility for your tablet, or your tablet dealer.