In 2016 the IRS sent me a $309 bill for unpaid 2015 taxes. I paid the bill in 2016. I'm just now doing my 2015 and 2016 taxes using TurboTax. Where do I enter the $309?

You do not enter it in any return unless your state has a deduction for Federal taxes

The money you paid to the IRS for a previous year’s federal tax return is not deductible, so you do not enter it anywhere on your next federal return.  If you paid taxes due  to your state or local government, you can enter that by going to Federal> Deductions and Credits> Estimates and Other Taxes Paid> Other Income Taxes.  You can enter the taxes you paid, but do not include any penalties or interest.

There are six states that allow you to enter the federal tax you paid on your state return--Alabama, Iowa. Louisiana, Missouri, Montana, and Oregon.  If you are filing in one of these states watch for the deduction when you prepare your state return.