You've enabled Online Archiving for a 365 user and a few days go by and the user complains of a lack of space. This can be solved by manually starting the archiving process in PowerShell, this process is normally on a weekly schedule.
In PowerShell run: Connect-ExchangeOnline
Once logged in run: Start-ManagedFolderAssistant -Identity "user@domain.com"
If you don't have the Connect-ExchangeOnline command run the following in admin PowerShell. I usually have this in a connection script so the module stays up to date.
Install-Module -Name ExchangeOnlineManagement
Update-Module -Name ExchangeOnlineManagement
Import-Module ExchangeOnlineManagement; Get-Module ExchangeOnlineManagement
Connect-ExchangeOnline -ShowProgress $true
