For security purposes the user's email address has been encrypted.
This means there is another InventoryLab account that is linked to the Merchant ID they are trying to sync.
If they do not know the email address, you can ask for the Name associated with the inventoryLab account or the Merchant ID. If they do not know the Merchant ID it can be found under Settings>Account Info>My merchant Token.
If the previous account has not been used in a while then you can click the checkbox next to the old account and select Remove Merchant ID.
If it has been used recently then they would need to subscribe with the old account.