For your question, I tested in postman and it ran successfully, below I share my testing process.
First, we need to grant the app the delegated permissions (OnlineMeetings.ReadWrite) it needs, as shown in the following diagram, and use a work or school account.
Then we need to use the auth code flow to get the token, after getting the token, first parse our token in the jwt.ms, make sure the token has the permission we granted OnlineMeetings.ReadWrite, as shown in the image this is my parsing result.
Finally, we can use that token in Postman to send a POST request, create an online meeting, and my test results are as follows.
Hope this helps.
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