Hi, Olleke,
Thanks for visiting the Apple Support Communities. If we understand, you would like to enter your 6-digit device passcode when accessing the Passwords pane in Settings, instead of using Face ID. We are happy to provide some information!
When using Face ID, the feature is automatically enabled to access certain settings on your iPhone, including Passwords. While you cannot automatically default to using the 6-digit device passcode here, the phone will suggest using the passcode if Face ID fails two times. If Face ID fails twice and an incorrect passcode is entered, Passwords will not be accessible.
If you'd like to use just a 6-digit passcode for Passwords in Settings, you will need to disable Face ID on the iPhone entirely.
For more information on Face ID, check out the following Apple Support articles:
Use Face ID on your iPhone or iPad Pro - Apple Support
About Face ID advanced technology - Apple Support
If you'd like to see additional options related to Face ID and passcodes in a future iOS release, you can let Apple know here: Feedback - iPhone - Apple
Have a great weekend!