I have a capital gain from 2017. I also have carryover losses that more than cover for the capital gains. Why is TurboTax indicating that I still owe money on my capital gains? Is there an additional US tax that will not be offset by carryover losses?
Are you certain it is the capital gain that is being taxed? Even if your capital gain is offset by carryover losses you may still have taxable income from other sources (wages, interest, retirement plan distributions, etc.). And, yes, there are other taxes besides the regular income tax, such as self-employment tax and the additional tax on an early distribution from a retirement account, which wouldn't be affected by carryover losses.
You can preview your 1040 to see the types of taxes on your return.
If you're using the TurboTax CD/Download software, you can use Forms Mode to preview your 1040.