I have the same problem--something to do with Turbo Tax coding, in the 2020 version as I have had no problems in the prior years .
I was able to go to Schedule S smart work sheet from my 2020 CA Tax return and followed these steps
DoubleTaxed Income Smart Worksheet fill column "b" with the amount from the other state-- leave other columns as "0" zero
In the Table Schedule: other Tax Credit, under A fill Tax paid to other State and B fill gross income taxable by other state
This seems to give the correct amount of Tax credit and cleat the final error check , allowing e filing. Not sure if this is an " acceptable" fix--BV
