Hi @Thomas_TeamDR,
Welcome to Box Community, and we’re glad to assist!
The message prompt that your Admin has received is a reminder that your Admin is about to add an existing Box user.
To resolve this, please have your Admin to follow these instructions:
Add a managed 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.
This will send the user an email invite as well as a message in their Box account that they've been invited to join your Box enterprise account. Once the user accepts the invite they will be rolled into the enterprise.
- Web Application Pop-up:

- Email Invitation:

For more, you can visit this article.
Hope it helps, and if you have questions please let us know.