"My mother is a waitress at a Chinese restaurant."
I am hearing this is the case.
People are speculating that Duo is attempting to teach the use of the particle 的 more than the best native sounding sentence structure; omission could be unnecessarily confusing for some absolute beginners. Not too mention the course is only in Beta.
We won't really know until a course contributor responds.
Any reason why this shouldn't be accepted? It seems more correct than the "correct" translation. She in a waitress AT a Chinese restaurant. While the "correct" translation seems to imply she is a special type of waitress that only works at Chinese restaurants.