Hi there,

Welcome to Box Community!

You should be able to manually add the personal account, please disregard the message "Email already used in a Box account but not associated with an enterprise". You just need to click the save option twice.  After your first click, a red banner will appear that indicates failure. Press the save button once more and you'll get the prompt that says "1 user added". Then, the user will receive an email invitation to join the enterprise.

I trust this information proves helpful in resolving the problem. 

Thank you for posting!

Hey Ann,

Thank you for the suggestion, but it did not work. When I press save the second time I get "User cannot be added" and the "Save" button gets stuck loading and won't allow me to select it again. 

Can I please get an update here? This issue is not resolved.

I'm experiencing the same issue and need resolution ASAP.  I've tried the suggestions above without any change in results.

I am experiencing the same issue.  Can anyone help?