"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.
https://ttlc.intuit.com/replies/3300757
