8829/SALT error message

My apologies if this has been asked and answered, but I keep receiving an error message that line 11b on form 8829 is in error because SALT amounts for real estate tax deductions are limited to a combined total of $40,000. I checked the Form 8829 for both our Schedule Cs, and the proportion of real estate taxes deducted is equal to the percentage of the home office space (33%), and the itemized deduction on Schedule A is 67% of the real estate taxes we paid last year. Why am I receiving an error message then? Why would the SALT cap have any impact on a Schedule C deduction?

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