You may not have biometrics login working if you didn't authorise its use when you first logged in.
On iPhone :
Go to phone settings
Search for Bilder
Face ID (or Touch ID)
For Android (the process may vary from device to device):
Go to phone settings
Choose App settings
Click All apps
Choose Bilder
Enable biometrics
Once activated in the phone settings, go to the app settings and enable the feature.