If you have after-tax basis in the plan, the Simplified Method must be used to determine the taxable amount when the payer indicates that the taxable amount is unknown. If you have no after-tax basis in the plan, which I don't think is the case, the entire amount is taxable.
When you've used the Simplified Method with TurboTax in past years, when you transfer in your previous year's tax return to begin your current year's tax return, TurboTax normally carries your previously provided Simplified-Method information forward so that you don't have to enter it again. I don't know why that would not have happened in this case. If necessary, review the TurboTax's 1099-R worksheet for your CSA-1099-R to find the details.
