Won't let me enter Social Security Tax Withheld

I've entered everything for my w2 - though  line 4 - Soc Sec Tax W/H. shows 000.00. The box is tinted gray (unlike the other boxes). When I try tying in the numbers nothing happens. If I highlight the 0.00 and hit delete - nothing happens. I've quit the application and re-opened it - same thing.

 

Thoughts?