You can only deduct any state taxes (for any year) or state estimates actually paid in 2019 on your 2019 federal return if you itemize deductions. But just the tax, not interest. You can not deduct any back taxes on a state return. You can not deduct prior year Federal Taxes paid on a federal return, no matter what year they are for.
Enter it under
Federal Taxes (or Personal for H&B)
Deductions and Credits
Then scroll way down to Estimates and Other Taxes Paid
Income Taxes Paid - Click the Start or Update button
Next page scroll down to Other Income Taxes Paid in 2019
Balance due on your 2018 State taxes paid in 2019
or Payments for 2018 or a prior year state or local paid in 2019
