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It sounds like the 2-factor authentication was enabled on this Ipad.
So it takes MORE than just your PIN number to unlock it now. It takes 2 things now...hence the 2 factor authentication.
This is ONLY turned on by the USER, and is NOT turned on by ANY OTHER MEANS.
What im saying is.....somebody (whether it was you, unwittingly....or it was somebody else, again unwittingly) has to turn the 2-factor authentication on through the settings menu.
The updates DO NOT enable this...unless you click on YES during the initial setup to turn on 2-factor authentication.
So.....now that we have established the problem.....it might be VERY hard to remove the 2-factor authentication if you DONT have the credentials to do so.
Its a security measure....and it was easy to bypas, it would not do any good...Soooo....you MAY have to take it in to a repair shop, and have an Apple technician RESET the Ipad manually.
But first.....what happens when you click on the "Didnt get a verification code"?
Oh, and the verification code is sent by TEXT message to a phone....they dont call you......just FYI