"It's not one o'clock."
ではありません is a single word, not component pieces, the individual parts do not serve independent purposes. They are just the parts of a word. I guess duo is just trying to get you to spell it out to pay attention to the spelling.
I suppose that it may be another word that used to be a whole phrase (like こにちは) which maybe be why it used the character ha to create the wa sound, but I am not sure.
But, either way, it is just the negative form of です. So if you are speaking politely and you say something is something else you would end the sentence with desu and if you are saying it isn't something then you would end with dewaarimasen (but with the character ha instead of wa).