In order to use Face ID/Touch ID or Fingerprint, you first need to enable it in your phone setting. For both iOS and Android, go to 'Setting', then select 'Touch ID & Password' for iPhone, or 'Security' for Android.
Next, tap 'Account' menu of the app, turn on the Biometric ID option for your device and enter the password for your Online Merchant Account.
By enabling Touch ID login, you are allowing anyone who has a fingerprint stored on your device to access your Merchant Account. Please review the fingerprints stored on your device and make sure all are authorised to access the Account information through the Amex Merchant app. You can turn these features on or off any time via the Account tab at the bottom of the screen.