When you log into the app for the first time, after having entered your 4 digit code, a message will pop up saying "Would you like to use Touch ID to login?"
Simply tap "Use Touch ID" button at the bottom of the screen, and this will enable the feature. From now on, you'll also have the choice between using your Touch ID or your 4 digit code when logging into Albert.
If you want to de-activate or reactivate Touch ID, tap the profile icon at the top left-hand corner of the Albert home screen, and then scroll down to the 'settings' option at the bottom of the menu. From here, you'll be able to adjust the Touch ID toggle according to whether you want to keep using Touch ID or not, as shown in the screenshot below.