Hi ​@hvs0909, 

Welcome to Box Community and we’re glad to assist! 

The message “Email already used in a Box account but not associated with an enterprise” typically occurs when you try to add or invite a user whose email address is already linked to a personal Box account, not yet connected to your enterprise.

Root Cause:

• The user has an existing personal Box account with that email.
• The email is not currently associated with any managed enterprise account.

Here’s the resolution to your error, 

1. After receiving the error “1 user failed to be added,” ignore it and press Save again.
2. This will send an invitation email to the user and a notification in their Box account.
3. Once the user accepts the invite, they will be rolled into your enterprise as a managed user.

For more details, you can visit this article. 

Thanks for posting!