Why is my1040-SR's Box 5a is empty when Box 5b is not?

[VolvoGirl]

Because it would be the same amount.  If 5a is the same as 5b leave 5a blank.

[VolvoGirl]

More details

If the taxable amount on 5b is the same as the gross distribution on 1099R box 1 then it doesn't show the amount on 5a.

See IRS 1040 Instructions page 26 middle column
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf

It says........

If your pension or annuity is fully taxable, enter the total pension or annuity payments (from Form(s) 1099-R, box 1) on line 5b; don’t make an entry on line 5a.