This box is asking for the outstanding principal on the mortgage as of January 1, 2020. First you might try leaving the field blank. If that doesn't work, can you use $0 or if not, use a different positive number like 1.00 which fits the below description? If so, then that would be more accurate.
The description of the amount to be reported in box 2 is as follows:
Box 2. Shows the outstanding principal on the mortgage as of January 1, 2020. If the mortgage originated in 2020, shows the mortgage principal as of the date of origination. If the recipient/lender acquired the loan in 2020, shows the mortgage principal as of the date of acquisition