To my ears there's something inherently off in this translation.
Seems to me the meaning here is closer to, "let's learn (or study) so that we know something". But, of course, this was considered incorrect. Can anyone explain?
While your translation should have been accepted (and will going forward), there really is no other natural way to put the English sentence here into Czech than the way it is. In other words, Czech cannot be without some , abychom, . že. so even when those are skipped as is natural in English, you do need to put them into the Czech sentence to make it whole.
Other possible Engish translations:
1. Let's study in order to know something.
2. Let's study so that we may know something.
It is a difficult sentence to translate into English. I'm a native English speaker, and understood the Czech, but used the word bank to see what English answer Duolingo might accept. But that's no criticism; when learning new languages, we need to deal with words and phrases that defy direct translation.