There could be several reasons your 1099-R distribution is not taxable (or has a non-taxable portion).
If you received a 1099-R for Retirement Income such as an Annuity that you made after-tax contributions to, there can be a non-taxable portion of your distribution.
If the amount in Box 1 differs from the amount in Box 2a, with an entry in Box 5, that would probably be the case for you.
If your 1099-R indicates a Rollover with a Code G in Box 7, that is not taxable either.
TurboTax asks follow-up questions after you enter your 1099-R to determine type of income and whether taxable or not, so pay attentions to those entries, too.
Click this link for more info on How to Enter a 1099-R.