A backdoor Roth is a traditional IRA contribution followed by a Roth conversion. It does not involve a recharacterization of the traditional IRA contribution. When entering the traditional IRA contribution, answer No when asked if you recharacterized (or "switched") the contribution.
If TurboTax is flagging an excess contribution, either you mistakenly indicated that you did such a recharacterization or you separately entered a Roth IRA contribution.
