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Make sure that nothing is on your TrueDepth camera or face until you set up the Face ID. Face ID is built for touch and glass use. face recognition works best if your iPhone or iPad is about 10-20 "(10-20") ahead of your face.
You will be asked to use a passcode to verify your identity as an alternative, if you have not set a passcode.
For the option of using a face ID or resetting Face ID, go to Settings > Face ID & Passcode. Settings are given.