If this is for the self-select PIN, it is not required - you can use your AGI from 2017. TurboTax does not have that number.
The Self-Select PIN was a random 5-digit number that you entered as your electronic signature when e-filing your return last year. It is not required for 2018.
The IP PIN is a 6-digit number that the IRS assigns to victims of tax-related identity theft.
If you have other questions about this, ask in the comment section below.
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