TurboTax fails to ask the questions related to distributions from an inherited account if the account is in an employer plan like a pension or a 401(k), but that's not important if the beneficiary is the surviving spouse of the distribution was not rolled over.
A blank box 2a is supposed to mean that the payer does not know how much is taxable and they are also supposed to mark box 2b Taxable amount not determined if the taxable amount is unknown However, some payers mistakenly leave box 2a blank when they know that the taxable amount is zero, and this would be suggested if 2b Taxable amount not determined is not marked. Still, it's unusual for a code-4 distribution from a qualified retirement plan like a pension or a 401(k) to be entirely nontaxable, so I would contact the plan to find out if the blank box 2a was meant to indicate that $0 is taxable.
