You are correct that your ending mortgage balance is not reported on your 1098 form. The box 2 amount of mortgage principal from your 1098 is the mortgage balance from the beginning of the year (Jan 1, 2018). Therefore, the ending mortgage balance asked for is the balance as of the end of the year or December 31, 2018.
You will need to find the ending balance either on your mortgage statements or by contacting your mortgage company.
You are receiving
the request to enter your ending mortgage principal balance due to the change in tax
law related to the mortgage interest deduction. There are now different
mortgage interest deduction limits based upon when the mortgage was originated
and what it was used for, so portions of a mortgage listed may include interest
that is not deductible. Therefore, the ending mortgage balance is needed
to determine the deductible portion of the interest being entered.
Here is more
information about the tax law and the provisions for now being able to deduct
mortgage interest:
https://ttlc.intuit.com/replies/3300473
