When you try to use this command:
"/fill 28 64 14 20 64 14 unpowered_comparator 2 replace air 0"
That doesn't work, the game says :
"Invalid tag encountered expected '{' as first char"

But if you use this command (upside down):
"/fill 28 64 14 20 64 14 air 0 replace unpowered_comparator 2"
It works.