"No one will know that I am a doctor."
Translation:Niemand wird wissen, dass ich Ärztin bin.
Keiner is the Singular masculinum nominative form for the indefinitepronoun. The pronoun can be in all numerus and so you can create a plural Keine (keine werden wissen), but this is nearly never used: Its just possible to create this inflection, but in 90% of the usages, like in this example where we have no noun, you use Keiner the masculinum singular form and in 10% the femininum singular form (when talking about scenarios, where everyone is female). When you have a noun, then the Plural is seen pretty often (or as often as negations of plural nouns are used)
To translate "no one, nobody [in the world]", you use "keiner". Like "no one" and "nobody" in English (and "niemand" in German, too, which is more standard than "keiner"), it has singular properties ("nobody knows" - "keiner weiß", "Keiner mag kalten Kaffee!" - "No one [in the world] likes cold coffee!").
...which is because (like MortiBiRD said) it's masculine singular, with "keine" and "keines" the respective feminine and neuter forms, and "keine" the plural.
But you only use feminine, neuter and (rarely, I think) plural forms when referring back to a noun, not to translate "nobody". Example: "Ich habe fünf Katzen (die Katze = feminine). Keine (= feminine singular!) mag Milch." "I have five cats. None (= not one) [of them] likes milk."
It is a waste of time reporting - DuoLingo is not interested in being corrected. Witness this link which goes back years. And the further you go, the worse it gets. They are American and use their language WHICH IS NOT ENGLISH - only a derivative. Unfortunately, their German translations get worse the further one gets. Yes, Duo. Contact me and I will correct your English for you. L E P