I found the tax penalty mainly comes after I input $12000 in box 2a, if I leave it blank, everything is fine.
Those $12000 are totally after-tax, and it should NOT be taxed again.
For the 1099-R which summarizes the distribution of $12000 ($6000 from 2019 & $6000 from 2020 traditional IRA to Roth IRA conversion).
It has $12000 in box1, $12000 in box 2a, checked in box2b, code 2 in box 7 and IRA/SEP/SIMPLE box checked.
