1099-R from deceased grandparent There is an amount in box 1 blank box 2a, box 2b only Total distribution checked, same amount in box 5 as box 1 dist code D4

No other boxes checked, turbo tax calculates $0 got letter from IRS saying I should have been taxed on all of box 1 though from Tried re-entering as amended, but still says I would owe $0
on amended 2021 taxes. 
Received 1099-R on distribution from deceased grandparent who was 99 in 2021
Box 1 $Amount
Box 2a Empty
Box 2b Tax amount not determined is UNchecked
Box 2b Total distribution checked
Box 5 Same as Box 1
Box 7 code D4