VK API - why does the photos.getMessagesUploadServer method display error 15?

Question

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There is an application VK Iframe.
From the backend, I turn to the photos.getMessagesUploadServer method and constantly display error 15: Access denied: no access to call this method, although I pass the access_token from the GET request to the frame into the request. In the same way, I turn to the photos.getWallUploadServer method - everything is ok there.

Answer 1

All because

This method can be called with a user access key obtained in the Standalone application via Implicit Flow.

as it is impossible to work with messages in iframe applications.
And photos.getWallUploadServer is possible, since wall posts can be sent from an iframe