Mortgage Int Statement (Form 1098) shows a $0.00 balance in box 2 "Outstanding mortgage principal as of 1/1/2018." Turbo Tax will not advance without an amt. Why?

The outstanding principal had been paid off by 1/1/2018, but the HELOC (line of credit) was still in place to be used later, which I did. And it incurred interest for 2018 as shown in box 1 on the Form 1098.