Very likely that you don't have the Gate Seal or have a later gate sealed and haven't re-opened it. As for the Academia AF 4XX Gate Seal, it's in Oerba AF 300. Head to the winding stairs and stop at the middle landing, turn away from the ocean and the treasure box is just a bit above, so throw Moogle at it.
"If sense were common, everyone would have it." The gamer who dislikes FF VII, OoT and similar and isn't afraid to admit it