The calendar account can’t connect to the server.
For example, you may not be connected to the internet or there may be a problem with the server.
The invitee hasn’t responded to your invitation yet, but he or she is busy during the time requested.
The invitee declined the event or shared calendar invitation.
A red badge on the Calendar icon in the Dock shows how many new notifications you have for invitations and changes to events and shared calendars.