No - mucho is either in context. It's just a gap in Duo's database for this question. The owl has been favouring a lot, lots of, etc this version so it just forgot this time. I think "much" sounds better in this sentence actually but "a lot" should be accepted. I hope everyone else is reporting it also.
Duolingo has many variations for its answers, you just need to get one that's in its database. (do not, don't, will not, won't). They also include both English and American e.g. Mum or Mom, shop or store, specialise or specialize, even though they use the US flag to depict the English language.