The Accelerator Add-in grayed out in Outlook desktop (Explorer view)
If you are using Outlook for PC/Laptop or Outlook for Mac, the add-in will be grayed out if the Reading Pane is turned off or hidden.
Ensure that the Reading Pane is turned on by following this Microsoft article for Outlook for Windows and this Microsoft article for Outlook for Mac.
If the Office 365 add-in in Outlook for PC remains grayed out in both your main Outlook inbox ribbon and in your Outlook message ribbon there is an issue with your Outlook software itself. Try closing Outlook and reopening it.
If this fails try repairing the outlook install.