The report server cannot open a connection to the report server database. A connection to the database is required for all requests and processing. (rsReportServerDatabaseUnavailable)
occurs when the SQL SERVER instance, to which SSRS is pointed to, is down or a very silly mistake, the SSRS is not at all mapped to a SQL SERVER database as happened in my case.
You can map the SSRS to a SQL SERVER database by using the Reporting Services Configuration Manager.