"I am a waitress."
The choicinator or whatever it's called is telling me Я є офіціантка is a correct answer as well - how does that work?
Correct, but no one says "Я є офіціантка.".. You only really use "є" to clarify that something/someone has something ("У квартирі є спальня." - "There is a bedroom in the apartment."). If you describe something/someone being something/someone than you usually leave it out ("Я - офіціантка.")
Sorry, it was added by a contributor that wasn't aware of the UkrDuo policies.
Or, it was added long ago and never removed. The current policy is,
In this course it's not accepted because
1) It is not natural in standard Ukrainian
Using "є" is obsolete in modern Ukrainian and sounds unnatural. It is pretty common in some regions of Ukraine though, but not in the "textbook" Ukrainian.
2) Copying the structure of the English sentence
The idea is, if a learner actually does not speak any Ukrainian, they might write "є" simply because they are copying the structure of the English sentence. The developers of this course think that the users should learn that it's unnatural, even though it's technically not wrong grammatically. And if the users do actually speak some Ukrainian with the variation/dialect where "є" is natural (e.g. some Western Ukraine regions, Canadian born Ukrainians), they can understand that this course is also for people who don't speak Ukrainian at all and just go along with it, speaking the way they want at home, and following the "textbook rules" here :)