I'm working on my 2020 & 2021 taxes. I did a 401K withdrawal for the COVID relief & forgot to put my 2020 taxes. Can I put it on my 2021 taxes?

You can only do that on the 2020 tax return to spread the withdrawal from the 401(k) over three years due to Covid.

you can't report a 2020 1099-R on 2021 tax return.

You must report a 2020 1099-R on 2020 tax return.

To divide by 3.0 is optional.

For most simplicity check the box to opt out of dividing by 3.0.

@earl_walker2003- 

It's odd that the IRS has not sent you a letter billing you for a tax deficiency for underreporting the 2020 401(k) income.

fanfare's suggestion for opt out of splitting the income over 3 years would result in a higher late-payment penalty than would choosing on 2020 Form 8915-E to split the income evenly over 3 years.