The tax table is only one of seven different methods that can be used to calculate your tax, depending on what is in your tax return. Your tax return must require one of the other methods. The most common reason that you cannot use the tax table is that your income includes qualified dividends or long-term capital gains.
In the IRS instructions for Form 1040, look at the instructions for line 16. Starting at the bottom of the right column on page 32 it explains the seven methods that can be used to calculate the tax on line 16.
If you are using the CD/download TurboTax software you can see which method TurboTax used by looking at the 7 checkboxes in the Tax Smart Worksheet. You can see the Tax Smart Worksheet in forms mode. It's between lines 15 and 16 of the Form 1040 or Form 1040-SR Worksheet.
