"Mój ojciec jest rolnikiem."
Tłumaczenie:My father is a farmer.
We use an indefinite article with professions, like in the sentences:
He is a farmer,
He is a cook.
Using a definite article in the above sentences is incorrect.
You cannot say it's incorrect because grammatically it's just fine and you could easily find a situation in which it may be used. You automatically tend to use the idefinite article because there is no context, but the idea on Duolingo is that there is a context, we just don't know it. Otherwise many sentences won't make sense.