What do I enter? If my outstanding principal mortgage is zero, system won't let me enter that on form 1098.

 
KrisD
Intuit Alumni

Deductions & credits

You can leave Box 2 blank if you enter a 2018 origination date in Box 3. 

The program won't like it, but if you enter a 2018 origination date in box 3, you can continue and the error will be removed.