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:

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

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