At the end of last year Microsoft decided to disable the feature to copy meetings inside the Outlook calendar.
To re-enable copying meetings set the following registry key:
[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\Calendar]
"EnableMeetingCopy"=DWORD:1
Restart Outlook after setting the registry key and everything should work again.
More information: https://support.microsoft.com/en-us/office/outlook-blocks-copying-meetings-with-copying-meetings-is-not-supported-4baaa023-2199-4833-b7ac-d9f0715d50f1