Things are a bit confusing because, unlike English, Portuguese typically does not use "a/an" with occupations. We say something like "I am an engineer" but the Portuguese equivalent "Eu sou engenheiro" is literally "I am engineer" and "Eu sou um engenheiro" sounds slightly odd to a native speaker. That's why Duolingo's translation is "Eu sou especialista em vinho" without "um/uma".

As you point out, the other confusing thing is that sometimes the name of the profession changes with sex and sometimes it doesn't. For example a male engineer is an "engenheiro" and a female engineer is an "engenheira", whereas both a male and a female journalist are a called a "jornalista" ("especialista" is like that and there is no "especialisto").

Although I said that "um/uma" is not typically not used with professions it can be added when you do more than simply state your job, for example you might want to say "I am an excellent journalist" which is "Eu sou um/uma jornalista excelente". The choice of "um/uma" depends on whether you are a man (um) or a woman (uma) even though the word "journalista" looks feminine due to the final "a".