I think you're missing the point, which is that a lot of the times when people would use "entonces" in Spanish, people would use "so" in English. Some dictionaries, like this one:
even have "so" as an accepted translation, and give examples.
I think it would be a valid translation and I think it would improve DuoLingo to offer it as a hover-over hint and accept it in contexts like this. The German course does this a lot. You can't learn a language effectively just by learning correspondences of literal meanings of words, because they don't correspond to each other in a 1-1 way, and "entonces" is a good example. Sometimes it means "then", but other times it's very casual and it's just a word that starts the beginning of a sentence, and thus might be natural to translate as "So".