Same issue here, not a Steam player though - I purchased the code through the link within my mogstation, as I'm a player with a US credit card but a European service account (cause that's where I lived when 1.0 came out) I wanted to make sure I get the right one.

They can't transfer my account to the US and it's an issue every time, and now I get this error, which seemingly has to do with using a US IP. Tried VPN, tried everything, the code I already paid for is simply being rejected, even though the pre-order code worked just fine. Why do we need two keys again, Square Enix?

No response from support so far... If need be, I heard the Square Enix Store is very fast with refunds if you mention that error code.