After an iOS/iPadOS update, you need to use the passcode first.
After that, Touch ID should work as usual.
More info:
To start using Touch ID, you must first set up a passcode on your iPhone or iPad (or a password on your Mac). You must enter your passcode or password for additional security validation:
- after you restart your iPhone, iPad, or Mac;
- when more than 48 hours have passed from the last time you unlocked your device;
- to add or delete a fingerprint to use with Touch ID;
- to change the iPhone or iPad passcode or Mac system password, and for other security settings like FileVault on your Mac;
- when there have been more than five unrecognized Touch ID authorization attempts in a row; and
- after you log out of your Mac.