Why can't I uncheck a prefilled check on Box 13

You can try deleting the W-2 and re-entering it without checking the boxes.  

To do this in TurboTax Online you can follow these steps:

1. Within your return, click on Tax Tools in the black menu on the left side of the screen.
2. Click on Tools.
3. Click on Delete a Form. 
4. Scroll down to your W-2 and click Delete.
5. Under Federal in the black menu bar click Wages & Income to get back into your tax returns.
6. Using the magnifying glass at the upper right of the screen, search for Form W-2 and select the Jump to link.
7. Enter your W-2 according to the form you received.

To do this in TurboTax Desktop you can follow these steps:

1. Within your return, click on the Forms icon in the upper right of the screen to switch to Forms mode.
2. Scroll down in the menu bar on the left and select your form.
3. Click the Delete Form button at the bottom left of the preview screen.
4. Confirm the deletion then click the Step-by-Step icon in the upper right of the screen.
5. Using the magnifying glass at the upper right of the screen, search for Form W-2 and select the Jump to link.
6. Enter your W-2 according to the form you received.

If you imported the W-2 or if you received it by other means and that box was checked, you need to leave it checked. If it's not supposed to be checked, then you need to contact the employer ASAP to have them issue a corrected W-2. Otherwise, not entering the W-2 "exactly" as printed could result in issues with the IRS. Not sure if that particular box would cause an issue. Just can't rule it out.