"The English man is a musician."

Translation: Az angol férfi egy zenész.

[DocMelVis]

Why is it not correct to add van at the end of this sentence?

[jsiehler]

Predicate nominative sentences like this ("X is Y" form) don't use van or vannak.
There's some explanation in the Notes:
https://www.duolingo.com/skill/hu/Basic-2-alternative

[Elon_the_Hittite]

Apparently not when considering someone's occupation. And despite the presence of the indefinite article in the answer, I omitted it, in line with occupations in other languages, and it was accepted.