Your 1098 gives the beginning mortgage balance for 2018. Turbotax also asks for the ending balance. This is not on your 1098, you will need to get it from your mortgage statement or lender's web site. It should be less than the beginning balance.
This is needed because, if your beginning OR ending balance is more than $750,000, your deduction may be limited, and a special calculation is needed.
One problem is that the program does not seem to ask this during the 1098 interview, at least for some customers, so it only comes up during the error check. Another problem is that the program doesn't tell you where to get the number or why it is needed. (We are pointing this out to the moderators.)
If your beginning and ending mortgage balance are both under $750,000, then it is probably OK to estimate your ending balance as a few thousand dollars less than the beginning balance (but it really isn't hard to find the ending balance from your lender).
