I figured it out. I had to double-click on the field and enter the value directly into the tax form (not the TurboTax step-by-step box). Turns out I had a net operating loss in my 2018 return, so TurboTax was indicating I qualified for a State of MI property tax credit based upon my negative taxable income. That was not valid, as the State of MI doesn't recognize a NOL as reason to lower one's income to qualify for what is normally a tax credit for lower-income people. So, I had to delete the primary residence property tax value from my 2018 return so it didn't flow to the State of MI return. IT was OK since I took the standard deduction. However, that locked up the input field in the step-by-step process and carried over to TurboTax 2019. Not sure why. I hope it isn't going to be that way in all future years!.
