While there is this support article about Touch ID and workaround, should it fail to work;
not certain how or why it hasn't functioned, according to directions in your instance.
• If Touch ID isn't working on your iPhone or iPad - Apple Support
https://support.apple.com/en-us/HT207537
"If the previous steps didn't help, you see "Failed—unable to complete Touch ID setup", or
either of the following happen when you go to Settings > Touch ID & Passcode, take your
device to an Apple Retail Store or Apple Authorized Service Provider, or contact Apple Support:
- You can't turn on Touch ID
- Touch ID is dimmed or grayed out
[The instructions cover variables, including these; while I'd guess you have already
visited the support page above, because your post here includes the URL within it.]
"You might need to enter your passcode or Apple ID instead of using Touch ID in these situations:
- You just restarted your device
- Your fingerprint isn't recognized five times in a row
- You haven't unlocked your device in more than 48 hours
- You just enrolled or deleted fingerprints
- You're trying to open Touch ID & Passcode in the Settings menu
- You used Emergency SOS."
[If you have not read this companion article, please read for more info.]
Because your topic appears in 'macOS 10.13.6 High Sierra' community
not sure how these steps may, or may not, have functioned correctly.