This is probably caused by the database not being fully caught up, while your package is technically ready to download, permissions updates haven't updated fully, permissions are updated every night.
A manifest file is a file that is contained within a package that has the express intent of serving as an extra layer of metadata for that specific package. Manifest files tend to contain various forms of data and then a S3 link to the file associated with that data, this effectively allows advanced users to perform specific operations on specific files within a package without having to operate on the entire set of data.