If the error above shows when a super admin is trying to assign a license, that means the user is already part of another organization.
A common cause for this is if the user is currently using a trial license of the software. The trial license will have to be deleted by the super admin and then a license can be assigned out through the Admin Console.
When deleting the trial account, a prompt will come up saying it will take up to 15 days before the account is completely deleted. An email will be sent to the user once the deletion is completed.