"The entered code has already been registered"

[Deproxy]

You need to go to: https://store.na.square-enix-games.c...ccount#/orders

Login and click Unlock code. The main game code never come from your email.

Yes, I tried to do precisely that, but was thwarted by the migration to the new store, which ended up wiping my previous orders. I sent the Store Support an e-mail and they sent me an e-mail with a link to the page I should be getting by doing the procedure you said, which is the first screenshot. They then told me that if the code I'm getting on that page is not being accepted at the MogStation, I should send an e-mail to another support team, which is the one I narrated in my opening post.