"You are a big help for me."
Translation:Vi estas granda helpo por mi.
Certainly in (British) English it is more usual to say, "You are a great help to me." Duolingo accepts "to me" when the Esperanto sentence is presented for translation into English.
In a noun phrase, the adjectives must be declined the same way as the noun. If the noun takes the plural -j, the adjectives take the plural -j. If the noun takes the accusative -n, the adjectives take the accusative -n.
So you have two things wrong: First, "granda" and "helpon" are not declined in the same way. Second, the verb "esti" does not assign the accusative. It's a stative verb, not an active verb. What follows is not a direct object but a subject complement.
The correct answer is "Vi estas granda helpo por mi."