How do I calculate the \\\"outstanding mortgage principal\\\" when I bought my house in Sept 2024?

Do you have a Form 1098 and is there an amount in Box 2?    The entire mortgage is considered principal on the day you purchased the home.   TurboTax is asking for the total amount mortgaged when you purchased the home.   

TurboTax needs to know if you have more than $750,000 in mortgage debt since that is the maximum amount that interest can be deducted on.  See - How much mortgage interest can I deduct?  

No, I do not have a 1098. I bought the house under land contract from my grandparents. 

The amount of debt you owe is your outstanding balance.

Since it is seller financed, that is the option you select. You enter the interest you paid on the loan to them and they enter the interest received on their return.