Make sure you federal tax withholding was properly entered into TurboTax when you entered your Form 1099-R.
If it is properly entered, it is likely due to your income tax bracket (you did not withhold enough based on your income tax bracket). Many things affect your income taxes such as other income, what deductions you have, filing status, dependents., etc.
In general a 20% Federal withholding is taken from a retirement distribution. It is not an exact amount that will necessarily cover your taxes owed on your distribution.
You can view your entire return or just your 1040 form before you e-file:
Click here for additional information on why you may owe income taxes.