@eva01 , I assume that this was not a rollover of a Coronavirus-Related Distribution or any other type of disaster distribution, just an ordinary rollover. The rollover you did was a permissible taxable rollover and is not permitted to be undone. If the Form 1099-R from the 401(k) plan did not report this as a taxable rollover and you did not report this as a taxable rollover on your 2020 tax return, you must amend your 2020 tax return to show this taxable income by submitting a substitute Form 1099-R that shows the correct taxable amount in box 2a. Being a permissible rollover, there is no 10% penalty but the IRS will almost certainly bill for interest and maybe an underpayment penalty.
