I just got one of these letters for my 2015 California State taxes also! My reason says "You may not deduct income on the Schedule CA that you did not include in the adjusted gross income shown on your federal income tax return." But I'm the same as you, I've always used Turbo Tax and transferred the federal info to the state forms, no errors found. They want almost $700 from me which I do not have! I was a fire victim in 2017 and still recovering, plus two young kids, all on one income. I don't know what to do, but this is incredibly frustrating as time is a rare commodity for me to dive into this and figure it out, not to mention my 2015 taxes burned in the fire...
