You are probably using a username or email address that is already present in our database. For security reasons, we do not allow the same username or email address to be linked to more than one HubbleClub account, not even when in combination with a different email address.
Just try to use a different username/email address, one that is more likely to be unique, not just Michael or Michelle, for example. Please also note that the username should consist only of one word.