@wbbagnall
what probably occured is this:
the states get a data feed from the IRS a few years after the tax season ends, so CA knows what your 2019 AGI was from your federal tax return. Since you stated you lived in CA "all year" (because of the form your filed), CA expected the CA tax return to match the Federal Return. It didn't. Hence, the letter.
There is a 4 year limit to claim a refund in CA. So it is doable, if you can find the software to do it. Depending on the complexity, maybe it can be done manually?
https://www.ftb.ca.gov/help/disagree-or-resolve-an-issue/claim-for-refund.html
