If you're receiving the "INVALID_COUNTRY_CODE" error, this is likely because the account has not set up a Country Code yet.
To resolve this issue, please visit the Nexon Website and log in with your account credentials. You will be prompted to assign a Country Code to the account. Once this has been selected, you will be able to access the website and be able to log in to the Nexon Launcher and / or Steam without experiencing this issue.
Please read more about the Country Code here: Country Selection FAQ
If you are still experiencing an issue, please submit a ticket with the ticket title "[INVALID_COUNTRY_CODE]".