For 2021 1040 line 15 is $4K. Line 24 is $0. According to my reading of the tax tables, line 24 should be 400, 10% of 4K. What am I missing?

there are multiple ways to compute taxes.  if you have qualifying dividends or long-term capital gains they may be taxed at zero.     we can't see your return. so there could be other reasons. 

What tax is on line 16?  Anything listed on lines 17-24?  

See the IRS worksheet on 1040 page 36 for how the tax is figured.  Turbo Tax uses the same worksheet.

https://www.irs.gov/pub/irs-pdf/i1040gi.pdf

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.