I tested your issue in graph explorer and I didn't have the same issue as you, my advice to you is to first make sure that you have delegated permissions for the app OnlineMeetings.ReadWrite, and if you're using an app you created yourself, you should use auth code flow gets the token and in the jwt.ms parses whether the token you got has the permission OnlineMeetings.ReadWrite.
I recommend you test it in graph explorer first to see if you still run into the same issue. When testing in explorer, you need to log in as a user and then grant the OnlineMeetings.ReadWrite permission. This is shown in the figure below.
More information about the API can be found at this link:
Hope this helps.
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