Translation:We pack our suitcases before our departure.
saying "we pack", meaning present tense, when you're talking about something that seems to be in passed tense, sounds pretty odd. I mean...even in french that seems odd at least to me.
Duolingo marked "We pack our suitcases before we leave" as wrong, but that correction overlooks a fundamental difference between French and English: French more frequently uses a nominal style (favoring nouns over verbs). The phrase "before our departure" is perfectly correct, but more formal, in English, than the everyday "before we leave" (or "before we go"). I agree with Carl549388, and Sitesurf, but it is also important to point out the underlying principle.
What do you mean by more formal?
You seem to be an advanced student. Keep in mind that most of us are still learning basic vocabulary words. It's smarter for us to stick with the actual vocabulary words that we are being taught. You are more on the doctoral level, while I'm struggling with pre-K. Have you tried the language forums on sites like WordReference? You could have a great discussion there!