Luckily, every gaffe in a Star Wars movie can either be fixed with computers or explained away in some kind of ancillary product. In this case, it's the latter. It turns out that multiple comics went out of their way to remedy Chewie's injustice. Back in 1980, even the Marvel Comics series that thought giant green bunny rabbits were super cool had enough common sense to realize the movie dropped the ball here. The comic introduced the idea that Chewie was given his medal after the ceremony, because of the embarrassing indignity Leia would have to endure standing on a table to put it around his neck.
Similarly, a more recent Chewbacca-centric comic confirmed that he got a medal, but explained that he didn't wear it because medals "clash" with his "warrior vibe" -- so he gives it away to some kid. Which is at least slightly less crappy than popping by the intergalactic pawn shop.