I answered “saya punya pertanyaan” and it marked it as correct but it said there is an alternate answer which is the same just with a capital S on saya and punctuation. Why?
That is a sentence. To start a sentence, the beginning of the sentence must be capitalized and end with full stop punctuation. That's why Duo suggest us to use the capital S and end the sentence with full stop punctuation.
"Saya punya pertanyaan."