You cannot deduct federal back taxes anywhere. The IRS does not allow it, unfortunately.  However, you can deduct state back taxes, they are part of your itemized deduction. 

1. Federal Taxes
2. Deductions and Credits
3. Scroll down to Estimates and Other Taxes Paid
4. Select Other Income Taxes
5. Select payments for 2015 or prior year state or local income taxes paid in 2016