Since you are requesting a sensitive scope from Google, you need to have your calendar project verified with Google. First of all make sure you add a Calendar scope to your project in google dashboard, open OAuth consent screen and click on “Add Scope” button:

Start typing Calendar, and select Google Calendar API from the list, the select ../auth/calendar from a scope list, this will allow your website to edit view calendars and edit events on the agent’s calendar:

Now you can click on “Submit for verification” button and follow instruction Google tells you.