Look at these equivalent sentences:
Il est avocat.He is a lawyer.
Marie est avocate.Marie is a lawyer.
Je suis vendeur dans un grand magasin.I am a sales assistant in a department store.
Paul est médecin.Paul is a doctor.
In French we don't say un or une (a) when stating someone's profession.
We literally say "He is a lawyer", or "She is a doctor."
Also note: the job description changes if the person is feminine or masculine (see gender of professions)
In French, you always need an article in front of a group including an adjective, and that rule applies to professions:
Lisa est une très bonne vendeuse.Lisa is a very good sales assistant.
Maurice est un professeur excellent.Maurice is an excellent teacher.
Vous êtes un mauvais avocat.You are a bad lawyer.
See also Job titles differ in French depending on whether you're a man or a woman (nouns)
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