Enter "$1" as the "outstanding mortgage principal" amount (Box 2, Form 1098 Mortgage Interest) on the original 1098 (the one you paid off with the refinance).
This will not have any effect on your tax calculation and will not generate an error in either TurboTax or with the IRS, From an economic perspective, it is accurate - you owed virtually nothing on this mortgage at the end of 2020.
Why not enter "0"? There are many places in TurboTax where you must make a non-zero entry, primarily to let the program know that you answered the question and that the answer makes some sense.
Why not leave it blank? The IRS requires an entry in Box 2 of this form.
In this case, entering "$1" tells the program that you didn't forget to answer the question and that you had at least some amount of a mortgage loan balance to justify claiming an interest deduction.
@gaoch
