How does TT calculate the taxable portion of permanent disability income?

How does TT calculate the taxable portion of my permanent disability income? Received CSA-1099R form for FERS disability retirement income. Box 2a Taxable amount states- "unknown". Regular monthly payments started January 2021.

Following steps and answering questions in TT premier as I go through 1099-R section/inputs but TT keeps arriving at incorrect (at least I think is incorrect) answer. TT concludes, zero of gross distribution in Box 1 is taxable? I do have federal tax taken out of my monthly distribution. and Box9b has an amount for Total employee contributions.

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