Are you age 70 1/2 or older? Is the distribution on the Form 1099-R from an IRA? If so, did you click on the box IRA/SEP/SIMPLE? If so, then after you have entered the 1099-R there will be a series of screens where one will ask Do any of these situations apply to you? You should have selected I transferred all or part of this money directly to charity
While entering your form 1099-R, if you have entered a birth date which makes you older than 70 1/2 in the tax year, TurboTax will ask you if any portion of your distribution was transferred to charity. You can then enter the amount transferred to charity.
The code in box 7 of form 1099-R must be 7 and the box IRA/SEP/SIMPLE must be checked.