You have a hybrid deployment of on-premises Exchange Server and Exchange Online in Office 365 in which primary mailboxes are located in the on-premises environment and archive mailboxes are located in Exchange Online. When you try to move only a primary mailbox from the on-premises environment to Exchange Online by using the Exchange admin center in Office 365, you receive the following error message.
Error: MigrationPermanentException: You must specify the PrimaryOnly parameter
The Exchange admin center doesn't have the option to move only a primary mailbox. To do this, you must use the New-MoveRequest cmdlet.
To move only a primary mailbox, follow these steps: