Form 8962 is a very complicated form, so don't feel alone in your frustration.
If line 25 is larger than line 24, there wouldn't be an entry on line 26, and there should be a positive number on line 27. The instructions for line 27 say " If line 25 is greater than line 24, subtract line 24 from line 25. Enter the difference here." So, if line 25 is larger than line 24, and you subtract line 24 from line 25, line 27 will be a positive amount.
This means that you'll owe a repayment. However, what you owe might be limited -- see “Table 5 -- Repayment Limitation” on page 16 of Instructions for Form 8962
(2017).
