Deductions & credits
You should enter the amount of the HELOC taken on 1/6/2020 in Box 2, even if it is not shown on your 1098 form. This is required so that TurboTax can figure out the correct amount of interest you can deduct on your return. There is a limitation of mortgage interest deductions with total loans above $750,000 (if originated after December 15, 2017).
I am not sure of your exact tax situation or the total of your loans, but I will explain this further so that you understand why Box 2 is required for mortgage interest calculations and why the beginning balance is required (rather than the ending balance).
ILLUSTRATION:
Deductible mortgage interest is calculated by taking the loan limit ($750,000) divided by your outstanding loan balance (Box 2). Then, that percentage is multiped by the mortgage interest as shown on the 1098. This amount is then deducted on Schedule A.
I created a mock-up return for further illustration. As shown in the picture below - the total mortgage balances inputted were 900,000 with 50,000 in interest. 750,000/900,000 is .8333 (as shown on Line 15 below). Then, this is multiped by the mortgage interest inputted to get the total deductible mortgage interest for Schedule A.
In addition, as per the chart shown in Publication 936 (2020), in order to fully deduct your mortgage interest, it must be under the limit at all times during the year. Generally, the largest balance is at the beginning of the year (Box 2), which is why the information is required for correct calculations.
I hope this helped answer your question on why you are getting an error for not inputting your beginning balance in Box 2.
