"Taxable amount unknown" is the message that you get when the custodian of the plan changed after the account was first created. When the amount in Box 2 of your 1099-R is blank or lists "unknown," leave that box blank when you enter the 1099-R in TurboTax. TurboTax will then want you to tell it how much was taxable. If there is an entry in box 5 or 9b, TurboTax can use those amounts to calculate the taxable amount. If those boxes are blank, and you don't know how much you contributed in after-tax money to your state pension, then you will have to assume that the full amount is taxable.
See this link for the TurboTax FAQ on entering taxable pension income.
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