"In my opinion, we must quickly do something."
Translation:À mon avis, il faut vite faire quelque chose.
Their meanings vary slightly. “Falloir” is used to express necessity while “devoir” means something is an obligation or duty. In that case, the verb “falloir” is more accurate to use in the sentence. The sentence indicates something that needs to be done not whether a person must do it or not.
Check it here: https://www.lawlessfrench.com/grammar/devoir-vs-falloir/
I think it's because of Duo wanting the emphasis on "must" = il faut rather than "have to/need to" which is more the meaning of "devoir" in this context, but I'm not an authority on this. See: https://www.collinsdictionary.com/dictionary/french-english/devoir If you back-translate your version Reverso gives: "We need to do something fast." and Collins gives "We need to do something quickly."