If you found this Q&A by search engine, you may also find it helpful to visit the more thoroughly discussed topic at Google Calendar API v3 Access Not Configured
I had to manage my project, first by visiting https://console.developers.google.com/project.
I clicked the Edit allowed referrers button and added these to the list:
localhost localhost/* *.localhost/* *.localhost 127.0.0.1/*
At the time of writing, this was all done under APIs & auth > Credentials. This is necessary even for publicly available calendars.