I think there's an error. It played me the audio, and I typed it correctly, exactly the same as the "correct solution".
That's possible that it happened, but this report is useless.
What did it play? What exactly did you write? What did it say that was the correct solution? Best thing to do is to make a screenshot with the error and the rejected and the correct solution.
Why než and not před? Is this short for dřív než?
před requires some object, before something. like "před tím, než zapomeneme."
You can view it as a short for dříve než. But it is not necessary. Než just can be used for temporal clauses.