On the tips & notes page for this skill (on web at least) it even listed どうも as "thanks", whereas for ありがとう it says "thank you".
So it's ironic that here the sentence we're being asked to translate is "Thanks." and yet it currently doesn't even accept「どうも。」for the answer! The default answer it wants is「ありがとう。」, which the tips & notes page lists as "thank you". XD
I think you're confusing どうもwith どうか. どうも is a polite way to say 'thank you.' It's more than a simple 'thanks,' which is what ありがとう means. But if you add ございます (meaning 'to be' or 'to exist') onto ありがとう, it becomes more polite. ありがとうございます. 'I am grateful,' or more literally, 'There is thanks'
どうも (doumo) - casual and informal
ありがとう (arigatou) - casual but polite; English speakers tend to misspell it as "arigato" because we don't pronounce long vowels - we spell とうきょう (toukyou) as "Tokyo" in English and we spell すもう (sumou) as "sumo"
ありがとうございます (arigatou gozaimasu) - polite