I sold my primary residence this year and received a 1099-S from the settlement company with Box 6 "Buyer's part of real estate tax". Where do I report this in TurboTax?

You would reduce the amount of real estate taxes that you paid in 2017 by the amount he paid to you in the settlement. 

The Buyer's part of the real estate tax is the real estate tax that the Buyer paid to you in the home settlement.   So each of you will only be reporting the tax on the home for the period of time you each owned it.

If I had just read the Instructions for Box 6 at the bottom of the 1099-S, it says the same thing.  So, I'll reduce the amount on the 1098, box 11, by the amount on the 1099-S, box 6, and report the reduced amount when I enter the information for the 1098 into TurboTax.  I'll also check the box for "this amount doesn't match what's on the 1098" and give this explanation of the 1099-S from the settlement company reducing the deductible amount reported by the mortgage company on the 1098.