My Form 1099-R has 0 in Box 9a. Turbotax is not accepting that value. How do I proceed?

Leave box 9a blank (empty) if the program will not accept a zero entry.  If you still receive an error after removing the zero in box 9a then delete the Form 1099-R and re-enter the form manually.

Don't enter a zero.  Leave it blank.  

In fact, the payer should have left box 9a blank.  Box 9a is only to be populated when there are multiple beneficiaries and the distribution is a total distribution.  A zero in box 9a is nonsensical because it implies that you received no portion of the distribution.  If you in fact received no part of the distribution, you would not have received this Form 1099-R at all.