Enable archive mailboxes for all users whose archive mailbox isn't already enabled.

Well known issue, that you can solve easily by running simple command in powershell:

Get-Mailbox -Filter {ArchiveStatus -Eq "None" -AND RecipientTypeDetails -eq "UserMailbox"} | Enable-Mailbox -Archive

Done :)

Please notice , to be able to do so you have to Connect to Exchange Online PowerShell.

 

This is as well useful option to Enable auto-expanding archiving