Box 12 should be entered into TurboTax exactly as it is on your W-2. If
there is nothing in box 12 on your W-2, leave it blank in TurboTax. When you
play with the box a bit, it thinks you have a value you need to enter. It
should read "-Select Letter Code-" for the letter code box and
the amount box next to it should be blank.
If setting box 12 back to "-Select Letter Code-" does not
work, then delete the W-2 and re-enter it being sure to leave box 12 the way it
is the second time.
https://ttlc.intuit.com/replies/3300234