"간장을 끓이고 고기를 넣으세요."
Translation:Boil the soy sauce and add the meat.
It might have to do with the use of the verb "to add". This implies that the two actions are related (not independent) & perhaps, 어서 may be more appropriate in such case, i.e.
간장을 끓여서 고기를 넣으세요. Boil the soy sauce and (then) add the meat. (2 sequential and dependent clauses)
2 sequential but independent clauses (actions):
간장을 끓이고 고기를 오븐에 넣습니다. Boil the soy sauce and (also) put the meat in the oven.
"Boil the soy sauce, then add in the meat" was marked wrong, but is more accurate than Duolingo's standard answer. Btw, since Duolingo is picky about literal translations, 끓이고 implies an after command. Thus more proper English would be to use, "then...", not "and...". If you want to say "and...", then the more accurate Korean would be 끓이면서.