As the error message reported: Migration is only supported in app-only context. Because Graph Explorer is usually used in user context, it uses user tokens instead of application tokens, so you cannot use Graph Explorer Tool to call the api.
According to the documentation, you must use the Teamwork.Migrate.All
application permission when you create a team in a migration state, so you must grant this permission to your application in Azure AD and then grant the admin consent.
Then you need to use client credential flow in postman to get the application token.
Finally, you will be able to use the token to call the api in postman.
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