Had this happen to me today, too, (after buying the expac back in May... >.>). If you purchased the expac from the Square Enix Store, (https://store.na.square-enix-games.com/ or whatever region you're in) if you log in to that website again, (may need to reset your password if you don't remember it - it's not the same as your regular game login / the Mogstation) in the upper right, you'll see your name, (/store alias) with a down-carat, (i.e. "^" but pointing down) next to it. Click on it, and "My Orders" and it'll take you to your purchase history, and for every expac you've purchased, there's a red "See Unlocked Key" button - press that and you'll be taken to another page that shows both your early access key, which you don't want, and your activation key, which you do. Open up Mogstation in another window / tab, and pretty sure on that initial "Service Account Dashboard" page, there's an "Enter your Endwalker key" (or something similar - doesn't show it anymore once you've done, so can't say precisely what it says, but it's an image of Endwalker) type in the code from the page with the two Endwalker keys and it should now show your account with a checkmark next to Endwalker. Unfortunately, it doesn't update your status while you're in the game, so you have to log out and go back to the end of the line yet again and/or try again another day if you can't get in at all, (*sigh*) but theoretically, at least, you should be up and running after this. If that's not the case when I finally get back in, (been a half an hour now and still have a long way to go before I decide to throw in the towel for another day) will update this with any further info. Good luck, (and holy heck, Square, it really is time to get your stupid back end systems to actually talk to each other directly - granted, this isn't anything like the nightmare of FFXI's back ends, but really - it is 2021 - time to do the work ;P ).