When entering into 2019 TurboTax as an additional estimated tax payment the payment made on 8/17/2021 and credited to 2019 by the IRS, simply ignore the error that TurboTax generates when you enter 8/17/2021 and jump to another part of TurboTax after clicking the Continue button to cause TurboTax to retain the amount entered. That "error" only prevents you from e-filing, which you can't do now with your 2019 tax return anyway. TurboTax will still apply the payment on Schedule 3 line 8. If TurboTax asks, tell TurboTax to let the IRS bill you later for any underpayment penalty.
Note that normally 2019 would be a closed tax year for obtaining a refund because the tax return is being filed more than 3 years after the April 15, 2020 filing deadline, but since you made a 2019 tax payment on August 17, 2021, the deadline to file a claim for refund is 2 years after that payment date, August, 17, 2023. However, under these circumstances your refund cannot exceed the amount paid within the 2 years preceding the date of the filing (which presumably is just the amount paid on August, 17, 2021).
