Precondition Failed explained
I am proud to explain it. The problem was that I tryed to write the sign for "percent" WITHOUT some number/s in front of it. You can try it:
write 1%, 11%, 20%, 99%, 100%...and it'll be okay, but if you try to write it without 1, 11, 20, 99, 100, etc., then you may see the Precondition Failed.
It was in our case last time. Probably there may be some other reasons for this Precondition Failed problem.