It's actually likely that it's the designated Roth account in the federal TSP rather than in a 401(k), but the answer is the same. Neither a 401(k) nor the federal TSP is an IRA. Enter amounts shown in box 12 of your W-2 only in box 12 of TurboTax's W-2 form, nowhere else in TurboTax. Under Deductions & Credits, go back and remove the Roth IRA contribution that you did not actually make.
