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Yes, you will have to amend your 2019 return to report the basis on Form 8606. Please How do I amend a 2019 return in TurboTax?.

To clarify, you stated "Roth conversion on 3/2/2021" for 2019 contribution but above you said you converted the amount in March 2020. Therefore, you had a conversion in 2020 and again in 2021, correct?

You also will have a Form 8606 for 2020 to report the change in your basis and the 2020 conversion.

To enter the nondeductible contribution to the traditional IRA, you will do this for the 2020 tax return and 2019 amended return:

1. Open your return 
2. Click on "Search" on the top right and type “IRA contributions”
3. Click on “Jump to IRA contributions"
4. Select “traditional IRA”
5. Answer “No” to “Is This a Repayment of a Retirement Distribution?”
6. Enter the amount you contributed
7. Answer “No” to the recharacterized question on the “Did You Change Your Mind?” screen
8. Answer the next questions until you get to “Any Nondeductible Contributions to Your IRA?” and select “Yes” if you had a nondeductible contributions before this tax year.
9. If you had a basis in the Traditional IRA before then enter the amount. (on your 2020 tax return you will enter the nondeductible contribution amount for 2019)
10. On the “Choose Not to Deduct IRA Contributions” screen choose “Yes, make part of my IRA contribution nondeductible” and enter the amount (if you had a retirement plan at work and your income is over a certain limit then it will just say nondeductible and trigger Form 8606)

To enter the 1099-R distribution/conversion: 

1. Open your return
2. Click on "Search" on the top right and type “1099-R”
3. Click on “Jump to 1099-R”
4. Click "Continue" and enter the information from your 1099-R
5. Answer questions until you get to “What Did You Do With The Money” and choose “I moved it to another retirement account”
6. Then choose “I did a combination of rolling over, converting, or cashing out money.” and enter the amount next to "Amount converted to a Roth IRA account"
7. On the "Your 1099-R Entries" screen click "continue"
8. Answer "yes" to "Any nondeductible Contribution to your IRA?"
9. Answer the questions about the basis and value