Try easiest first:
Hold home and power together until the phone reboots. Does it work? If not;
Power down the phone. Unhook the battery, then unhook the Touch ID cable on the screen and logic board. Clean all sides of the connection with 99% IPA. Use compressed air or blow them dry. Reconnect the Touch ID cable and then the battery. Then reboot your phone. Does it work? If not;
I heard a rumor that disabling Touch ID purchases in the App Store fixes this. To be honest, I have never had this error with a 6 and up, so I can't say for sure. But that is the rumor.