why not accept "cela ne fait rien"

Bonjour John ! Thank you for this useful remark ! Indeed, you can use "cela ne fait rien" in a more elegant way, however you will not be able to say "cela fait rien" as the omission of "ne" occurs in casual speech, and "cela" is used in elegant speech :) I've now edited the lesson and questions accordingly! Merci beaucoup et à bientôt !