"it's not a bug"? can anyone explain me
Sure! In software development, the word "bug" often means an issue with the software. Similar to "glitch" or "mistake". But also, "bug" means insect. So you could interpret the theme as "it is not a mistake", "it is not a glitch", or even "it is not an insect". Remember, the theme is not meant to be a strict rule. It is meant to help you think of creative ideas! So interpret the theme however it makes sense to you and have fun with it!