Hello , i have an iPhone 5S , and i have a problem with Touch ID. When i had iOS 7.1.2 , Touch ID wasn't even showing in settings , it said only passcode , and since i updated to iOS 8 ,Touch ID appears in settings , but when i want to turn on iPhon

Hello , i have an iPhone 5S , and i have a problem with Touch ID.

When i had iOS 7.1.2 , Touch ID wasn't even showing in settings , it said only passcode , and since i updated to iOS 8 , Touch ID appears in settings , but when i want to turn on iPhone Unlock or iTunes&AppStore and put my finger , it says "Failed , Unable to complete Touch ID setup.Please go back and try again." and i tried a lot of times and still not working..

Can you please help?