TurboTax added the underpayment penalty because the due date to file the return is also the due date to pay any taxes due.
However, when you go through the Underpayment Penalty information, you will come to a screen titled "Let the IRS Bill Me Later". On that page you can click on the button "Let the IRS Bill Me Later" and this will prevent the penalty from being added to the return. (See the screenshot below)
