1099-r for 12000. Taxability unknown. 6000 from 2021 paid first quarter of the 2022 and 6000 for 2022. Bkdr Roths. If you input 12000 6000 is taxable. Why ?

On the 2021 return you would have filed Form 8606 reporting the $6,000 TY2021 Non-deductible IRA contribution, establishing a contribution basis of $6,000.

On the 2022 return you report another $6,000 TY2022 Non-deductible IRA on Form 8606, bringing your total contribution basis to $12,000.

On the 2022 return you'll indicate the FMV of IRAs on 12/31/22 of $0.  That should allocate all of the basis to the $12,000 conversion amount on the 1099-R.

If your question is more along the lines of "how do I enter this in the software", that might be a question for @George4Tacks 

"6000 from 2021 paid first quarter of the 2022 and 6000 for 2022"

All at once? In and out, all at the same time? Let's remember that Backdoor is the same as Rollover/Conversion of basis and only basis. That's what makes it nontaxable.

"That should allocate all of the basis to the $12,000 conversion amount on the 1099-R."

What matters is:

When they put in any of this money, was there any other money in their Traditional IRA account that was the first destination? As long as there was no money in the Trad IRA each time they put in each $6k, then immediately converted (no earnings), you might simply have overlooked entered the basis, as noted.

"Bkdr Roths"

Plural?

If they put this into an existing or separate (not new)Trad IRA account(s), and/or there was any delay between contribution and rollover, then there is the consideration of pro rata computation against earnings, against previous pre-taxed amount, and other considerations the same as any conversion.

In other words, if they put $12k in one existing account and took $12k out, but there were other funds present from anything other than basis, you can't assign which $12k got rolled. It's pro rata.