I am having the same problem. I want to take the itemized deduction. My state accepts the federal itemized deduction and so my combined federal and state taxes are lower if I select itemized deductions. TT gives an error for line 12b. I would correct the error and TT indicates the error has been fixed. However, when you are about to file taxes it does a final check and automatically reinserts the $600 back at line 12b. TT keeps putting back the $600 in line 12b even when I correct it. This is clearly a bug. I've tried TT customer service twice and they are of no use. The problem is with the 1040 SR wks line 12b.
