Error:
["[Code:6024] [Error]You are not authorized to list products under this brand. To sell products under this brand, please try adding one of your SKUs under this brand through the 'Add a Product' page and follow the application instructions."]
Explanation:
This error is returned directly from Amazon via the API and if you were able to list items previously with that particular brand, but not anymore then you would need to contact Amazon or open a support ticket with them and see if they would be able to look into this and confirm if the policy on that brand might have been changed recently.