What is it asking for in Explanation for Mixed Straddle Account Election? I bought and sold double calendars for SPX and NDX and am filling out my Section 1256 loss.

If you've indicated a Mixed straddle account election, the IRS requires that you attach a statement to your Form 6781, specifically describing the class of activities for which a mixed straddle account was established.

 

From what you're saying, though, it's possible that you made an incorrect election on a previous screen in TurboTax.

 

If you selected Mixed straddle account election, rather than Mixed straddle election, which you might have chosen in previous years, an explanation will be required.

 

To see which election you previously indicated in TurboTax Online:

1. If you don’t see 2022 TAXES in the left pane, select the dropdown to the right of Income & Expenses on the Hi, let’s keep working on your taxes! page and then select Let’s get started, Pick up where you left off, or Review/Edit.  Otherwise, in the left pane, select Federal, then Wages & Income (This is labelled Income & Expenses in TurboTax Self-Employed)
2. Scroll down and select the Show more dropdown to the right of Less Common Investments and Savings 
3. Select  Start or Revisit to the right of Contracts and Straddles
4. Answer  Yes on the Any Straddles or Section 1256 Contracts? page 
5. You previously made your election on the On the Any Elections? page.  If your election should have been Mixed straddle election rather than Mixed straddle account election, select the box next to the first, and unselect the other 

Continue entering all information as appropriate.

 

See Mixed straddle election (Election A) and Mixed straddle account (Election C) in the https://www.irs.gov/publications/p550 for an explanation of each election type.