@Romanh 

"Can someone contribute to prior year Nondeductible IRA"

Let's separate these parts: Until the filing due date of the tax return, you can make an IRA contribution for that prior tax year. You might have limitations due to high AGI or being covered by a retirement plan at work and therefor be ineligible for contributing to Roth IRA directly. You might also be limited for being able to deduct your contribution to your Trad IRA, but you still might be eligible to contribution to a Trad IRA as a nondeductible (Basis) contribution.

"and convert it to Roth IRA before April 15?"

April has nothing to do with conversions. That’s a contribution deadline. Conversions and rollovers are actions not using new money. The contribution in a backdoor is when it went into the Trad IRA, not when it moved to Roth IRA.

Conversions are reported for the tax year of the date that action occurred. It is an event that happened for the year it was done in. You get a 1099-R for the money Out, as a conversion is first of all, a distribution. The issuer might not know what you did with it other than, it was removed from an IRA account.

"Would I have to create in his 2024 return 1099R with Distribution Code G?"

Code G might be taxable or not taxable. What matters is, was the money moved from one sheltered type to a similar one (such as Trad IRA to plan 401(k)) or a different one (Roth IRA from Trad 401(k)), and was there Basis or commingled funds. Remember, the issuer of the 1099-R only concerns their reporting for the money Out. Not for what happened next, or if there are commingled funds, or if there is Basis.

G: Direct rollover of a distribution to a qualified plan, a section 403(b) plan, a governmental section 457(b) plan, or an IRA. You need to know which action was taken next. That doesn’t seem like a backdoor Roth.

"Even if I don't have 1099R?"

Is that what he got, for tax year 2024? Or was the conversion done in 2025? You don’t make up a 1099-R. And a backdoor Roth IRA conversion only works if there are no pre-tax, deducted, funds in Trad IRA, SEP IRA or SIMPLE IRA. If the ending FMV of these accounts, aggregated, is other than $0, this will be a pro rata taxable conversion.

You get a 1099-R for money Out, for the event that happened in the tax year. If a backdoor is contributed in 2025 as a tax year 2024 contribution, then converted, the conversion (and 1099-R) are all in 2025. The Form 5498 for the initial contribution will be out in May and it should have been marked as for tax year 2024. Form 5498 is for money In. Your tax filing likely needs to include Form 8606 in both years.

"Then select box B4 Full amount rolled over to Roth IRA and put the amount contributed on line B8 Preciously taxed contribution?"

A Rollover is not the same as a Conversion. Which is it? Yes, a previously taxed contribution is Basis.

Are you using the 1099-R worksheet in ProSeries? Do you have a conversion, a rollover, a backdoor conversion, are you trying to report something in the wrong tax year?