What does it mean when I receive an error message saying "You must create your custom site address before users can log into their account"?
This error message means exactly what it says! We require that Client/Staff users login using a white labeled custom domain/sub-domain. Even if you don't intend to fully white label your dashboard, you MUST create a custom domain/sub-domain before you can create any Client/Staff users, and those Client/Staff users must login at that custom URL.
To configure a custom domain/sub-domain, simply click the link given to the right of the error message, saying "Setup Site Address":
Or, alternately, follow the instructions in this KnowledgeBase article.