How do I get to block 16 on my 1099R to enter my military retirement pay in block 16? It only brings up block 15.

If there are no state income taxes withheld in box 14 then leave boxes 14 through 16 blank (empty) on the Form 1099-R.

Are you sure that you are entering the military pension from DFAS as a plain 1099-R?

Certainly my software's standard 1099-R forms have box 16 showing in the main interview.

BUT...if you accidentally started entering the Military pension as a CSA-, or CSF--1099-R, then that  form (in the software) has no box 16

.......and if you wrongly entered your DFAS 1099-R as that CSA- or CSF-1099-R type of form, then you need to delete it and enter it as the proper form type.

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Certainly, if you have a US govt pension from the OPM, it is not a military retirement, and the form for that in the software would not have box 16....but a military retirement 1099-R from DFAS, is not a CSA-, or CSF-1099-R.