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

When I try and add a user to our Enterprise Account who has an existing free box account I get a message that states "1 user failed to be added". Creates a csv file that shows the email address to be added and the message "Email already used in a Box account but not associated with an enterprise". Also see a yellow circle with an exclamation point in it next to the email address of the user I am trying to add. Hovering over it pops up the same message.