steps to enable fingerprint password on iphone 7

See the suggestions in this link --> If Touch ID isn't working on your iPhone or iPad - Apple Support

The suggestions assume:

1. Your phone doesn't have physical damage, particularly, the Touch ID button
2. You haven't had the screen or Touch ID button replaced by anyone OTHER than Apple or an Apple Authorized Service Provider
3. If nothing helps, then it's time to schedule an appointment at your Apple Store Genius Bar for evaluation and options, but if it requires service, it really would be time to invest in a newer, more modern iPhone at this point.