Unfortunately, TurboTax does not support Louisiana's Form R-10606, Supplemental Worksheet for Credit for Taxes Paid to Other States for Forms IT-540 and IT-541.
However, the form and instructions are available here.
I also got a letter requesting the same form. I would think that a service that promotes itself as THE tax preparation software would provide the complete range of form that are used to calculate the return. It is not that unusual a situation to have income from more than one state. TurboTax has really dropped the ball here.
I also received the letter from Louisiana, no help on support either. My figures do not coincide with turbo tax. Guess they are not so good after all, will be sharing my experience with co-workers and friends.
Would someone please verify which figure from the Mississippi tax return goes in column 2 of the Louisiana R-10606. Is it line 16, Mississippi Adjusted Gross Income? Or is it line 19, Mississippi Taxable Income?