Touch ID and passcode
when I try to add a fingerprint a pop up menu appears and tells me that it failed
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If you're using a case or screen protector, make sure it doesn't cover the Touch ID sensor or the surrounding ring. Go to Settings > Touch ID & Passcode and make sure that iPhone Unlock or iTunes & App Store is on, and that you've enrolled one or more fingerprints. Try enrolling a different finger.
If Touch ID isn't working on your iPhone or iPad - Apple Support