Deductions & credits

....especially if you are using windows desktop software...as @Critter   mentioned....try a double-click. 

 

There is likely a "Supporting Details" worksheet attached to a field that shows "$0" and won't let you make a simple entry.   You can then either use the details sheet...or delete the sheet entirely and then use the normal field entry.

 

The same thing can happen in other areas in the program where you see a "$0" (and not just a "$" )where you can't make a simple entry

____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*