You should be able to use "it's not" or "it isn't" as well as "it is not" when typing out an answer. On the 'choose a tile' answers they may not include all those options though. Just for space consideration.
If you tried typing it and it was marked wrong, then that's a mistake and you should report it using the flag. Sometimes things like that can get missed and it has to be pointed out before they can change it.