You may see the following error message displayed in connection with the above message: 'The tenant for the selected user does not match the previously tenant selected'.

What does this mean?

The message you are seeing is produced when the Microsoft tenant you attempted to sync users from differs from the tenant used in a previous Sync.

How do I correct this?

If you intend to Sync users from a different tenant you will first need to disable Sync in the portal, then, when you next sync, be sure you are logged in to the correct O365 user account for the tenant you wish to Sync