You stated that the import mapping was automatically generated based on the needed attributes. When you created the JSON structure, did you use the exact response message or did you already take out some parts of the message before creating the structure?
I can't be entirely sure based on the image alone, but is it possible that the JSON structure was not created using actual response data?. For example, the correct relation could be
ProductView->extension_attributes->'maybe_even_another_level'->custom_attributes instead of directly using
Could you maybe post the sample response as a (prettified) text instead of an image?
UPDATE: The magento API documentation provides a sample where the item is called "CustomAttributes", but the actual response contains a container called "custom_attributes". Maybe Magento has forgotten to update the API documentation? Recreate the JSON structure using an actual response instead of the online documentation and see if that works :)
Are there customattributes objects present? If you do not have any customattributes there won't be an association too. You have checked security?
According to your mapping you're not calling for a description.
Even with the image link, it's really difficult to read. Is custom attributes coming back null? Have you tested this URI in Postman?