An amount being present on line B2 of TurboTax's 1099-R form indicates that you told TurboTax that you rolled that amount over to another tax-deferred qualified retirement account, either by indicating that you rolled over part of the distribution and explicitly entered that amount, or you had taxes withheld from the distribution, shown in box 4 of the Form 1099-R, you indicated that you rolled over the entire distribution, then told TurboTax that you did not substitute other funds to complete the rollover of the portion of the distribution withheld for taxes. If you did not actually roll any of that amount over, the IRS is correct that you filed in incorrect tax return and you owe the additional taxes.
If you did roll the money over to another qualified retirement account but it was to an account other than an IRA, the IRS receives no corroborating documentation from the receiving plan and often questions whether or not the rollover was actually completed. In that case you would need to provide to the IRS explanation and supporting documentation such as an account transaction statement demonstrating that you indeed deposited the money into a new qualified retirement plan as a rollover.
