If you already made the contribution non-deductible on line 1 of a 8606 form and did the conversion then it cannot be undone. You could amend to claim the deduction and then pay the tax on the conversion but there would not be any sense to do so (a lot of trouble to get a deduction that would be taken by the tax on the conversion - which is the whole point of a backdoor Roth - the non-deductible basis pays the tax on the conversion).
If you did NOT enter the contribution as non-deductible and do not have a 8606 line 1 amount then you did not do a backdoor Roth at all. If the conversion was done in 2019 the yiu should have a 1099-R for the conversion that will be taxable and the contribution should either be on the 8606 line 1 or a 1040 Schedule 1, line 19 ad a deductible contribution - if neither one then yiu failed to even enter the contribution and must amend to report it one way or the other.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
