I've finally figured out when and how this happens, even for situations when you have more than 1 share sold.
This happened to me because I sold a percentage of my original lot. When I selected that I received the shares from one grant lot, I entered the total shares vested/released and shares withheld, as a percentage of the original lot, as mentioned by the example.
Apparently, I didn't round to enough decimal places, and turbotax felt the math didn't add up. The easiest way to 'fix this is to go back to the 'how were you awarded the units you sold' screen, and temporarily select 'from different lots.
If your issue is the same as mine, the screen will say something like 'You must enter at least 0.2000 more shares to cover the ZZZ shares you sold'.
When I recalculated my shares with more precision, I realized that turbotax was correct. If you have the split screen open with the form at the bottom, as soon as you go back and correct the total shares vested/released, the red box goes away.
