Why can’t my iPad receive Apple ID verification codes
Why can’t my iPad receive Apple ID verification codes
iPad Air 2, iOS 8
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The iPad needs to be using IOS 9 or later. From the article below.
Trusted devices
A trusted device is an iPhone, iPad, or iPod touch with iOS 9 or later, or Mac with OS X El Capitan or later that you've already signed in to using two-factor authentication.
Two-factor authentication for Apple ID - Apple Support