It appears that you had an excess contribution and that you indicated to TurboTax that you wanted to withdraw more than the excess contribution was.
Is it possible that at one point you indicated that you would withdraw some or all the excess contribution, then later changed something else so that you no longer had an excess? Perhaps the system still remembers the amount you wanted to withdraw before the return due date (which is what 12B is), yet you no longer have an excess (which is 12A). 12C subtracts 12B from 12A but the result shouldn't be less than zero; hence why I think this is the cause of the error.
Sometimes TurboTax remembers a number but you can never get back to the original screen to change it.
The safest - although brute force - way to address this is to delete all HSA information and start over by doing the following:
1. make a copy of your W-2(s) (if you don't have the paper copies)
2. delete your W-2(s) (use the garbage can icon next to the W-2(s) on the Income screen
*** Desktop***
3. go to View (at the top), choose Forms, and select the desired form. Note the Delete Form button at the bottom of the screen.
*** Online ***
3. go to Tax Tools (on the left), and navigate to Tools->Delete a form
4. delete form(s) 1099-SA (if one), 8889-T, and 8889-S (if one)
5. go back and re-add your W-2(s), preferably adding them manually
6. go back and redo the entire HSA interview.