Hej,
Detta kan hända vem som helst. For example, someone already wrote about similar mistake in 2020: Bought wrong Region version - Support / Customer Support - World of Warcraft Forums
What most likely has happened here is that you accidentally bought an expansion for US version of the WoW account. If you have multiple WoW accounts, e.g., WoW 1, WoW 2 (lets say this is EU), WoW 3 (lets say this is US), always make sure that you choose the right one before making any purchases. As Trovlak said above, you can’t change your expansion from US to EU, as instructed by Blizzard here: Transfer World of Warcraft to a Different Region - Blizzard Support.
It’s not end of the world, as Blizzard is usually very understanding in case something like this happens - it could happen to anyone, very coommon mistake.
Use the Self-Service Refund, if not working, file a ticket. Once your refund has been processed, make a new purchase, but this time make sure it goes to your EU account.
Hopefully you’ll get it fixed!