The IRS letter states five reasons for why they say I owe more money. Three of the reasons have to do with claiming dependents (not applicable to me), one for AGI over $150K and the last for "computed incorrectly". If either of the last two are the issue, then Turbotax made the mistake and the error is in the their program. I, like everyone else, received IRS letter 6475 stating amount of Economic Impact Payment we received and that this amount must be part of your 2021 tax return to receive the RRC credit, which I did.
