Make sure that box 9a shows completely blank in TurboTax - do not leave even a "0" or a space in the box. The program reads a 0 as a value, so it will give this percentage error. If the box appears to be blank and is still giving this error, then try clicking in the box and deleting any potential spaces. If you imported the form, then you should delete the import and enter the values manually as there was likely an issue with the importing of your form.
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.
The IRS 1099-R form rules do not allow a zero in box 9a - any financial institution that puts in a zero is violating the form rules.
It is not an excuse, it is the published IRS 1099-R form rules. TurboTax has no control (or even cares) if a financial institution does not follow the IRS 1099-R instructions, TurboTax does and will flag an error when an illegal value is in box 9a, as it should. If you want to complain, then complain to the company that violated the rules, not the one that follows them.
There is only an entry on 9b if the distribution is split between two or more persons then the percentage that the person on the 1099-R is receiving is in the box. Zero is not a valid entry.