You mean that your employer admits that the D is for contributions to your 401(k) plan in 2018 and that D17 is for the 401(k) plan in 2017? Did your employer remove both years' contributions from Wages in box 1 (probably not)? TurboTax doesn't have an option for D17 because employers are not supposed to mix years on their tax documents. If your employer removed the 2017 contribution from Wages on your 2017 W-2 but forgot to enter the D amount in box 12, then they should have corrected the 2017 W-2 and you may have needed to amend your 2017 return.
Ask your employer if they really intended the D17 for the 2017 401(k) plan contribution, and if they removed it from Wages on your 2018 W-2...
