Line 5a shows the amount that is not taxable. It is only for reporting purposes. Line 5b is where your taxable pension amount should be placed.
If your pension amount is taxable, it will show on line 5b.
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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
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.