Make sure to add the following permissions of Microsoft Graph API in your app registration.
OnlineMeetings.Read.All OnlineMeetingTranscript.Read.All
Give Admin consent to the API permissions.
Second Thing:
You have to create an application access policy to allow application to access online meetings on behalf of a user.
Here are the commands to configure Application Access Policy:
Grant this Policy using your Azure UserID.
After creating and granting this access policy you should be able to use Graph API to access transcripts content using auth token of your app registration.
For Reference : https://learn.microsoft.com/en-us/graph/cloud-communication-online-meeting-application-access-policy
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