ProtonVPN 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 ProtonVPN website will not work on your InvizBox 2.
Click the following link to log in:
Once logged in you will find your OpenVPN credentials under "Account - OpenVPN / IKEv2 username".
You can find out where to enter these VPN credentials on your InvizBox 2 in this article.