wp-content/uploads/2013/07. Is its parent directory writeable by the server?
The parent directory is /wp-content. The error message is telling you that the process/user that WordPress is running as does not have permissions to create the “/uploads/2013/07 ” directories. If you temporarily change the permissions on the /wp-content directory to 0775 or 0777, and try uploading your media again, WordPress may be able to create the folders it needs to. Make sure you change the permissions on the folder back when you are done.