The creation of a user is linked to a license. Once you have started a project the system will not allow you to change it.
This is done to prevent the re-use of mailboxes for migrations rather than purchasing additional licenses.
To prevent incorrect mailbox id's being added to the system a verify process is automatically carried out when you add the users details.
If using a CSV file to import users please use the verify option from the main projects (Home) page before running your migration.
For further assistance in this area please contact our support team.