Android 7.1.2 is disabling some Pixel fingerprint sensors according to reports over on the Google Product forums. Though not all users with a Pixel and a Pixel XL on the most recent version of Android Nougat are having the problem, a number of users on the forums are already expressing issues with their fingerprint sensors no longer working post update. Some users are reporting the issue arising after receiving the update over-the-air and installing the downloaded software while others are reporting the same experiences of the sensor not working after sideloading the update through adb, so it would seem that it isn't just limited to those that received the update alert and downloaded it through the official means.
In addition to Pixel owners, it would seem that some Nexus users are reporting the same problem as well. In regards to Nexus devices the issue is affecting owners of both the Nexus 5X and Nexus 6P who have also installed the update. Some users of both sets of devices are reporting that the device is telling them fingerprint hardware is not available when trying to unlock the device using this method instead of inputting a PIN or password.
In this instance the first instinct might be to clear your cache and reboot the device, as well as delete the fingerprint and try to enter in new ones. According to users though the reboot and the clearing of cache data isn't helping the issue, and error messages are popping up that are stating users are prohibited from entering in new fingerprints after they've been deleted, so the only thing to do at this point might be to either wait for Google to find a fix, or try to revert back to Android 7.1.1 if that is a viable option. Unfortunately it doesn't seem like Google has a time frame on a fix yet, but it seems that they're at least aware of the issue being present. The silver lining perhaps is that the problem seems to only be affecting the unlocking process, with at least one user stating that using the fingerprint sensor's swipe gestures for bringing down the notification shade still results in the intended function being completed successfully.