I did not fill 8086 in last few years turbo tax returns. Recently I filed the missing 8086s to IRA. Now, in TurboTax online, how to enter prior year basis in 8086?

You can enter the prior year basis when you enter a traditional IRA contribution under Deductions & Credits or when you enter a Form 1099-R to report a distribution/conversion in the Income section. 

Please note if you do not get the “Any Nondeductible Contributions to Your IRA?” screen during the interview then please see Why am I unable to report my 2023 for 8606 basis? to sign up for updates on the issue.

In the IRA contribution section:

1. Login to your TurboTax Account 
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 contribution before this tax year.
9. Enter your basis in the Traditional IRA from your 2023 Form 8606 line 14 (if you had a basis in the prior year)
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 have a retirement plan at work and are over the income limit it will be nondeductible automatically and you only get a warning and then a screen saying $0 is deductible).

In the retirement income section:

1. Click on "Search" on the top right and type “1099-R”  
2. Click on “Jump to 1099-R”
3. Click "Continue" and enter the information from your 1099-R
4. Continue through the follow-up questions
5. On the "Review your 1099-R info" screen click "Continue"
6. Answer "Yes" to "Any nondeductible Contributions to your IRA?" if you had any nondeductible contributions in prior years.
7. Answer the questions about the basis from line 14 of your 2023 Form 8606 and the value of all traditional, SEP, and SIMPLE IRAs