Email already used in a Box account but not associated with an enterprise

Hi Jae, 

Welcome to Box Community and glad to assist! 

The “Email already used in a Box account but not associated with an enterprise" message means that the user you are trying to invite already has a personal account associated with that email address.

You receive the “1 user failed to be added” message so that you can acknowledge our warning message that “Groups and Folders selection, as well as User Settings for existing Box users, will be available after the user(s) accept their invite.”

We've already identified a solution to this error. Please follow the instructions from this article. 

Thanks for posting!