Line 15 is taxable income. It is just the total of lines 1-14. Do you not understand a line above?
Do you mean the tax on line 16?
If you have capital gains or qualified dividends the tax is not taken from the tax table but is calculated separately from schedule D. The tax will be calculated on the Qualified Dividends and Capital Gain Tax Worksheet. It does not get filed with your return.
In the online version you need to save your return as a pdf file and include all worksheets to see it.
