Norwegian does not put an indefinite article before professions, nationalities, and religions.
For example: Hun er advokat (She is A lawyer);
Han er amerikaner (He is (AN) American); Gutten er katolikk (The boy is A catholic)
It is often the small things that cause issue when learning something new. Spend a little time looking into this language point and I'm sure it will make sense to you very quickly. Lykke til!