"He is a waiter."
Translation:Él es camarero.
According to this article, the the "un/una" SHOULD NOT be used in as sentence such as the DL sentence. http://spanish.about.com/od/adjectives/a/indefinite.htm
Make sure you use the accent "él" means "he", but "el" means "the" and no, in Spanish they do not use the indefinite article with professions or nationalities. https://www.thoughtco.com/singular-indefinite-articles-3079098
It was reported sentence by sentence. You could report it as also correct at this sentence. The original lesson was teaching the word “camerero”, but “mesero” is used in Mexico. https://dictionary.reverso.net/english-spanish/waiter