Why is there no "un." Is this a mistake, or is thia the first case of not needing un/una?
It is a blantant attempt to expand our vocabulary. That is what is going on here. Horrors!
Yes, different countries use different words. “Mesero” is used in Mexico.
Spanish doesn't usually use an article in front of jobs, unlike English. So saying 'soy un maestro' sounds wrong. Think of it as a count. There is only one of you, so why say I am one teacher. English could be considered a bit redundant.
Your answer is not correct, because Spanish would not use “un” with an identity item such as a job title (He = waiter) and you must put the accent on “él” to mean “he”, because “el” without the accent means “the”.
If you are translating from English to Spanish, then you could report “mesero” as another correct alternative (used in Mexico) next time if you have fixed the other two issues.
In Spanish, they don’t put an article, but in English we have to, so it must be “He is a waiter.”
If "un" is not necessary could this also mean, He is THE waiter? Or would "él" be required in that case?