Is this the right place to get help? Need to be contacted by help desk, can't find answer to my problem on your forums, PLEASE HELP

Hi Réjean 

Welcome to Box Community and glad to assist!

You were prompted to this error because the affected user has an existing free account. To address this issue, add the user and after receiving the initial error message "1 user failed to be added", ignore the error message and press the Save button again. Saving a second time will begin the invitation process.

For more details, you can visit this article.

Thanks for posting! 

This does not work. When hitting save a second time, the save button only pulsates. 

This does not help either: https://support.box.com/hc/en-us/articles/360044195233--1-user-failed-to-be-added-Message-When-Adding-Managed-User

At the bottom of that support page, it has "9 out of 37 found this helpful". That is a horrible metric, and obviously not helpful to Box customers.