You need to enable static hosting for your application before you can upload and access content. You can enable static hosting from the Atlas App Services UI.
Enable hosting from the UI with the following procedure:
On the Hosting configuration page, click Enable Hosting. Atlas App Services will begin provisioning hosting for your application.
It may take a few minutes for Atlas App Services to finish provisioning hosting for your application once you've enabled it. You can upload content to Atlas App Services immediately, but you will need to wait for provisioning to complete before Atlas App Services serves your files.