Apple has temporarily stopped offering the iOS 9.3 update for older devices like the iPad Air and earlier and the iPhone 5s and earlier due to installation issues some users have experienced. On older devices, iOS 9.3 requires users to input the Apple ID and password originally used to set up the device, which can lead to the device becoming stuck at the Activation Lock screen if the original account information can’t be recalled.
Update: Apple has released a new build of iOS 9.3 for the iPad 2 and may be planning to roll out updates for additional devices. Apple has not yet resumed signing iOS 9.3 for affected devices.