If you see the error message above when you try to open a Google Calendar invitation, it's because Google strangely doesn't provide access to open calendar invitations for Google Cloud Identity (Free) clients. There's a simple workaround, however - simply right-click the calendar invitation link, and open in Incognito / InPrivate mode. This will open the calendar outside of your Google account, you should be able to accept the invitation, and receive the invitation in your (non-Google) email account.