I made nondeductible contribution 10 years ago and converted all to ROTH IRA in 2020. In 1099-R I received shows entire amount as Gross Distribution & Taxable amount and both box was checked. I don't have to pay tax twice on my nondeductible contribution. Can I override box 2a? I tried that, but it didn't make any difference on tax. How to report this?
No, please enter the 1099-R as shown on the form.
If you reported the nondeductible contribution on Form 8606 back then, then you can enter the basis when you enter your 1099-R. Then the part allocated to the basis will be tax-free.