This error means the person you are inviting is already a part of an organization or created their own organization by mistake.
Please let the person you are trying to invite know that they need to follow these steps to remove themselves from the organization.
Click on ‘More’ at the bottom right corner
Click on your profile name under ‘Settings’
Click on ‘Leave’ at the bottom of your profile.
Go the the Teams / People tab
Click on your profile name
Click on ‘Remove from organization’ at the bottom of your profile.