It is important to note that, the Face ID feature is available with the iPhone X version and later. The system was presented as the successor to Touch ID, Apple's previous fingerprint-based authentication technology embedded in the home button of the iPhone 8 and earlier devices in addition to the second-generation iPhone SE.
To use the Touch ID or Face ID to login with Full Slate, you will need to first log in with your email and your password successfully. After the first successful login, you will see the Touch ID or Face ID option in the password box. Make sure this feature is turned on in your settings.