Issues arising in each iPhone or iPad can be caused by different reasons and in different generations, such as iOS 8.1.3 or iOS 8.2 and also takes a different approach. Here is how to solve for iOS 8.3 that IT has been tested and nest success.
1. Go to Settings in your iPhone or iPad
2. Tap on the Touch ID passcode
3. Please enter the passcode or password (Password)
4. Please turn off (OFF) iTunes & App Store
5. Click the Home button to return to the original tab front (home screen)
6. Reboot off, press the Power button and shut down and open up again
7. Please enter passcode or password you set when first opened, it will Lock Screen
8. Go to Settings same as above
9. Tap on the Touch ID passcode
10. New type passcode or password
11. Please enable (ON) iTunes & App Store.
It's a simple way you can try to solve it yourself first if you encounter problems as described above. What you should avoid in order to avoid this problem is, do not use or Jailbreak Tweak your instructions without any professional in this section. Read instructions from any source that you trust before you decide to implement.