If your mortgage debt loans originated on or after December 15th 2017 and exceed the CA limit of $1,000,000 then an adjustment is calculated to reduce the interest deduction for CA.
If your mortgage debt should not be over $1M, or should not be adjusted because it was taken out before Dec 15, 2017 then perhaps you have entered more than one form 1098, such as for a refinance. That may have incorrectly pushed your total mortgage debt to over $1M for CA and/or the origination date for a loan is now after the Dec 2017 date. Go back through your form 1098 entries in the federal "Deductions & Credits" tab to be sure you have entered correctly.
I’ve called in multiple times about this topic and found a coding error in Turbo Tax backend spreadsheet. When you refinance it considers the “Average balance” to be the average of 1/1/19 and 12/31/19 which is wrong (let’s say 2mil and 0) which is 1mil. Unless you refinanced on July 1st this is always going to be wrong. The number above is way off, but my “total debt” is calculated by TT as more than the home value and at least some check should trigger an error. Can TT fix this obvious bug so users stop seeing the issue? I’ve talked to two CPA agents who both promise a ticket starting in March but the SW still fails.