Why isn't my pension income being entered on line 5A?

Is there an amount in Box 2a of the 1099-R?  If it is blank or says UNKNOWN, leave it blank.  Do not enter a zero.

Or, perhaps you accidentally checked that you did a rollover after entering that form.

Also, if the entire pension is taxable, it appears on Line 5b, and Line 5a is left blank.

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.

2020 Instruction 1040 (irs.gov) at p. 26

Thank you!

Line 5a on your 1040 is informational and appears off to the side of the form.  It's line 5b that's important, because it's the amount of your pension that is being included in taxable income.