Box 16 on the 1099-R is for state distributions. Many states require box 16 to be completed. If box 16 is blank you will enter the amount from box 2a. Box 2a is the taxable amount of the gross distribution. If box 2a is blank, and you haven't made after-tax contributions to the account, you can use the amount in box 1.
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My 1099-R Military Retirement DOES NOT have a box 16 and never has. You answered the question based on there being a blank box 16. Why now is your software asking for this information and won't allow completion of filing? I have been filing through Turbo tax for 23 years without this problem. Frustrated...