No mailbox with such GUID - ErrorNonExistentMailbox
This error is commonly caused by an unlicensed mailbox account. Check this before proceeding.
If a mailbox account has been assigned a license and the error still persists, it is usually caused by a non-Null value in the "msExchMailboxGuid" attribute of the mailbox account.
By default, when synchronizing a user from an on-premises environment that has a mailbox assigned to it, the value of the "msExchMailboxGuid" attribute will also be synced. Assigning an Office 365 license in this case will not result in the creation of a mailbox.
To confirm the issue in Office 365,
1. Log in to the Office 365 tenant using a Global Admin account, and go to the Admin page.
2. Under the Active Users panel, search for the email address of the affected user.
3. Click on the user name, and look under the Mail Settings.
4. The message associated with the mailbox will be :"This user's on-premises mailbox hasn't been migrated to Exchange Online. The Exchange Online mailbox will be available after migration is completed."
Follow the instructions in KB004336 to set the "msExchMailboxGuid" attribute to null. This will allow mailbox to be created in Office 365 so that MigrationWiz can proceed with the migration.