1. Check the email address of the calendar: Make sure you have the right email address for the calendar.
2. Check your calendar preferences: Go to File > Account Settings > Account Settings in Microsoft Outlook. Under internet calendars, check if the calendar exists in the list. If it is, make sure it is checked.
3. Check if the calendar is shared with the correct email address: Ask the person who shared the calendar with you to confirm that they have the correct email address. They should be able to update the calendar if they are the owner.
4. Check if the calendar is a different version of Outlook: Try to open the calendar on a different version of Outlook if you’re using the web version. The calendar may not be compatible with the web version of Outlook.
5. Check if you have the correct permissions: Ask the owner of the calendar if you have the correct permissions to add and sync the calendar. They will need to give you the right permissions if you don’t have them.
6. Check if the calendar is from a third-party source: Make sure the calendar is from a reputable source. If the calendar is from a third-party product, you may need to contact their support team to get help with adding the calendar.