Can't fix fingerprint on iPhone 6?

It’s a hardware problem, and fixing it would cost more than your 7 year old phone is worth. Did you have the screen replaced? That can break Touch ID if not done correctly, and is not repairable afterwards.

Did you try all the suggestions in the Related Article link above your post at the top of the page, which brought you here? If nothing worked, you can schedule an appointment at your Apple Store Genius Bar for evaluation. But honestly, given that iPhone 6 is a nearly 8 year old device, which hasn't been able to run the newer iOS versions for the past 3 years and isn't capable of running them, it might not be a wise investment to spend any money to fix the phone.