The most straight-forward way to do this is to delete all your HSA data and re-enter it. See "HSA Reset" below.
The reason for this is that TurboTax's screens are dynamic and that sometimes you can find it difficult to locate the same screen again (the screen where you indicate the amount you want to withdraw is one such screen).
So delete the HSA data and restart the interview.
***HSA RESET***
The safest - although brute force - way to fix this is to do the following in this order:
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.
