Usually the Taxable Amount on a 1099-R is the Amount in Box 1, unless there is a $0 actually reported in Box 2a.
If 'taxable amount not determined' is checked, TurboTax asks followup questions after you enter your 1099-R to determine taxable amount.
Here's more info to help you:
https://ttlc.intuit.com/replies/3300757
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