The game is region locked, but has only NA, EU, and JP regions. It sounds like yours is an EU account, in which case, you'd need an EU code for the expansion and wouldn't be able to use a NA code even if you got one.

I have no idea why you're having trouble logging into Square-Enix's EU store, though, nor why their sale wouldn't match between the different regions.

You could try just going to a third party retailer instead of SE directly. They won't have that particular 50% off sale, but Stormblood has been out long enough that they might have it cheaper than full price anyway. Just be sure that what you buy is for the right region. (If you get the game from a retailer, they'll give you a code which you then enter in the Mogstation to upgrade the account.)