How can you stop "Verification required" when installing free apps?
Generally verification comes up for one of two reasons:


Your previously-supplied payment information failed for your last purchase
You are using a device which has never purchased (or downloaded, for free apps) anything from the App Store with your account before.


Most likely it is the second one. Going through the payment verification process does not charge your payment method, it simply verifies it is valid. Once completed you can complete the download of your free app.