Hi,
Do you mean the envelope badge on the Outlook icon? If so, it seems like a bug, but you can try these steps to see if it works for you.
- Go to Windows Settings > Personalization > Taskbar > Other system tray icons
- Ensure Microsoft Outlook is turned on
- Under Taskbar behaviors on the same page, enable the option Show badges on taskbar apps
- In Outlook settings, ensure the option Show an envelope badge in the taskbar is turned on
- Unpin the Outlook icon on the taskbar
- Right-click on Outlook from the Start menu then select Pin to taskbar
- Restart the computer and see if it works