On my 2021 1040-SR return line 15 shows taxable income of $1,653. But on line 16 Tax Turbo Tax placed a 0 for the tax amount. Why would that be correct?

...hard to know without details about exactly where every source income came from.....especially if it was a mix.

For instance, if virtually all of it was from Qualified dividends, and long term capital gains...the tax rate could be 0%.    But once Interest and Retirement income get mixed in...not sure how it is figured as the tax tables are not used in either situation.

Probably there are details on a worksheet called Qualified Dividends & Capital Gain Tax Worksheet.