What's wrong with "È un centro industriale e commerciale importante"? Does the "importante" have to come before the noun?
No, it can go after as well; although it feels a little better before, I guess for symmetry.
I understand that when you have two adjectives for one noun, you place one before and one after the noun. In this case, I chose to put one before and two after and that is what the answer seemed to accept.