Surfshark use a different set of credentials to log in to their website and to use their VPN with OpenVPN. This is why the credentials you use to log in to the Surfshark website will not work on your InvizBox 2.
Click the following link to log in:
Once logged in you can get your OpenVPN credentials from the "Credentials" tab.
You can find out where to enter these VPN credentials on your InvizBox 2 in this article.