Many Steam users encounter the “Missing/Unavailable Content Manifest” error when trying to download or update Steam games installed on their PC. A Manifest is a list of files that contains the order of files and folders on the system. The Steam Manifest is loaded before the files are downloaded during an update or game download. Recall that game files are divided into several parts.
The manifest contains the order and type of files needed to successfully download or update a game. After downloading the manifest, Steam extracts the other game files and assembles them all into one file. If Steam cannot access or find the manifest, it essentially hangs and Steam users get a corresponding error.