Turbotax is saying I owe additional medicare tax but my income is beneath the threshold?

Go back and edit the W-2 you entered.  Look at the amount in box 6.  Makes sure that is exactly 1.45% of box 5.  If it is even $0.01 greater that will cause the program to give you this message.  Reduce the amount in box 6 by a few cents and the message should be removed.

The tax is an additional 0.9% percent Medicare tax on the portion of wages, self-employment, and railroad retirement income that exceeds a certain threshold.   That threshold is $250,000 for jointly filing couples, $125,000 for married couples filing separately, and $200,000 for everybody else.   It is based on all of your wages, it is not per w-2.