TurboTax has no ability to calculate RMDs. RMDs are whatever amount you tell TurboTax is an RMD. If a distribution was omitted from line 15, it means that either you failed to enter it or it got entered on line 16 instead (either because the RMD was from a retirement account other than an IRA or because you overlooked marking the IRA/SEP/SIMPLE box when you entered the Form 1099-R).
