Error sharing Outlook calendar
If you're trying to share an Outlook calendar that you own or made, you may receive an error, "Error while preparing to send message."
To fix this:
In Calendar view in Outlook, right-click the Calendar you're trying to email.
Click Properties
Go to the Permissions tab.
Remove any users who are no longer employees or whom should not have access to this calendar.
Hit Apply or OK and close this window.
Try re-sharing your calendar again.