Face unlock is not the most secure way to unlock your Android phone, but if you want to enable face unlock on Android 5.0 Lollipop and get the most of the Face matching feature, make sure your device is running Lollipop and do as follows:
1. First make sure you are already using a secure screen lock (Pattern, PIN, or Password)
2. Open Settings and go to Security (You can find it under the Personal settings)
3. To enable Face unlock, tap the Smart Lock option (You need to confirm you PIN, Password or Pattern to continue)
4. Select the Trusted Face option, and start setting up the face recognition feature by following the shown steps.
5. Once done, you just need to hold your Android device at the eye level, and the device will automatically unlock
Hi,see if this tip helps you get the Smart Lock enabled