No, I'm not trying to apply the code to a Steam account
Yes, I am trying to enter the correct type of code (20-digit code which I'm entering via Mogstation)
Yes, I bought the right region version (EU account, bought HW from the EU Square store)
No, I am not mistyping it (copy-pasted it from both the email and my account page on the store, which both give the same key)
Any bright ideas?