Why is this asking for beginning principal on mortgage? this information is not on my 1098

[Opus 17]

It is needed to calculate the deductible part of your interest, in some circumstances.  It should be in the papers you got when you bought the house (which you need to keep as long as you own the house plus 6 years, for situations like this).

If the mortgage balance is less than $750,000, a good guess will be sufficient in most cases.