To resolve this, ensure the users aren't already on another Box Enterprise or Standard account.

If they are, remove them from those accounts before re-inviting them to your Enterprise account. If the issue continues, contact Box support for further assistance in resolving 7 brew account conflicts

Hope! it would be helpful.

If you're seeing a message that the accounts are already part of a Box Enterprise or Standard plan, it means those users are linked to another Box organization. To resolve this reinstatement, the users can log into their Box accounts, go to Account Settings, then Account, and select “Leave Enterprise” if that option is available.