I am below the threshold for capital gains according to https://www.irs.gov/taxtopics/tc409 TT is still calculating that I owe capital gains

Long term capital gains are taxed at 0% if your tax bracket is 12% or less (less than $41,675 of taxable income for Single or MFS or less than $83,550 of taxable income for MFJ)..

However, long term capital gains are included in your taxable income for the calculation of your tax bracket, and that part of long term gain which crosses over $41,675 (Single or MFS) or $83,550 (MFJ) is taxed at 15%.

in effect, other income can make capital gains taxable. the calculation is done on the qual div and cap gain worksheet in Turbotax

here's the worksheet if you want to do a manual calculation to confirm what Turbotax has calculated

starts on D-16

https://www.irs.gov/pub/irs-pdf/i1040sd.pdf