"He does not eat pork, because he is vegetarian."
Translation:Li ne manĝas porkaĵon, ĉar li estas vegetarano.
“... li estas vegetarano.” means “... he is a vegetarian.” not “... he is vegetarian.”. In the English sentence given, the word “vegetarian” is an adjective describing him. It is not a noun.
It is interesting that I cannot report this, presumably because Duo accepted “vegeterana” as a typo rather than as a mistake.