Box 20 is blank on my W-2. What do I enter? During the final check, Turbotax is saying I must enter something.

[Anita01]

You wouldn't need anything in box 20 unless there's something in Boxes 18,19 and Associated State.  Id there is, then enter the locality name in 20 as it appears on your W-2.  If there are only zeroes or blanks or just a state code in the rest of the line on your W-2, then delete that unnecessary information.  It is there in error.