To get your PIN you can visit the IRS site Get an identity protection PIN (IP PIN). If you can't get the PIN online you can contact the IRS at 1-800-908-4490 or visit a Taxpayer Assistance Center.
Once you get the number, go back to your return and see the TurboTax help article to see how to enter it on your return:
How do I add or remove my 6-digit IP PIN?
