Normally, the only reason to do a back door Roth conversion is if you are not eligible to make a deductible contribution, because if you are eligible to make a deductible contribution you should also be within the income limits to make a direct Roth contribution.
At this point, you have to go back and file an amended return to report the contribution in the 2020 tax year because that’s when it occurred. I believe you can choose not to take the deduction, in which case you will generate a form 8606 to track your nondeductible IRA contributions. Or, you could take the tax deduction if you are eligible.
Then, since the conversion happened in 2021, you report that on your 2021 tax return. If you take the IRA deduction in 2020, you will get a tax refund from the 2020 amended return, and you will owe tax on the Roth conversion for 2021. Unfortunately, the amended return will take much longer to process, so you will have to come up with a tax money from someplace else for the time being. If you make a nondeductible contribution on your 2020 tax return, it will not change the tax that you have already paid, and then you will be able to report the conversion on your 2021 tax return. You need the form 8606 from your amended return in order to prepare your 2021 tax return correctly so you have to do the amended return first.
And very importantly, if you had traditional IRA balances already, the back door Roth won’t work the way you think it will and you will still owe income tax on the conversion. We can talk about that if we need to.
