Depending on what was reported on prior tax returns you may want to amend back to 2018.  On the 1099-R Wks I think if you scroll down to the bottom (might be "page 3") there's a worksheet you can use.  You need to know the retirement date.  Pub 721 is your "go-to" for this.  The worksheet on pdf page 28 is what the software mimics.  You'll have to manually accumulate and enter the "amount previously recovered" for the prior years up to the current year but it should carry forward with you once you have it in there.

https://www.irs.gov/pub/irs-prior/p721--2020.pdf

There's usually a 1-month lag in insurance so it's not always 12 months x monthly amount.  It'll be 11x + 1y = total.  That's one reason for the minor annual discrepancy.  I don't remember offhand whether the box 5 number includes supplemental insurance like dental, vision, etc. but I *think* it does.

Hopefully that gets you moving along.  No idea why the taxable amount would not be determined though, they're supposed to do that automatically now.