Note - Calendar sharing should always be done in the Outlook Web App Calendar, found here - https://outlook.office.com/calendar.
Attempting to share calendars via the Outlook desktop application has been met with more and more issues and failures ever since our mailboxes were moved into the Microsoft 365 Cloud Space.
Sharing a Calendar.
To share a calendar, browse to the calendar section of the Outlook Web App mentioned above - https://outlook.office.com/calendar
From there, identify the calendar you would like to share, click the ellipsis, and select Sharing and permissions.
Search for and select the user you would like to share the calendar with.
And choose the permissions you would like to give them.
After you click Share, your user will receive an email asking them to accept the invitation. They will not be able to see your calendar until they accept the invitation.
After that, the Accept will transition to View and they will be able to either use the link or view your calendar in their own calendar app. This calendar will eventually show up in their Outlook Desktop client, but it may take a little bit of time.