Zero is not allowed. The box must be blank.
(Box 9a is only used if the box 1 amount was split between 2 or more payees and tells you what your share is. A "0" (zero) would indicate that the 1099-R is reporting your share of nothing, which would be nonsense.)
Some payers insist on putting zero in the box to indicate that it is not used, when they should leave it blank according to the 1099-R form rules. TurboTax follows the rules and complains if it is zero.