Amateurishness is the noun: the quality of being not skilfully done or made. You point at the amateurishness of a drawing, a plan, or a performance. It is a long, rather formal word, the kind that turns up in a review or a teacher’s comment more than in chat. In speech, people usually just say it looks amateurish, or it was a bit amateur. The -ness does not add a new meaning; it only names the quality. Though actually, because the word is so heavy, using it can itself sound a little grand for a small fault — which is worth noticing when you write.