Looking at the documentation it seems to suggest that you now need to install a database engine yourself, or it defaults to a PostgreSQL install which is incompatible with our Enterprise Manager.Unable to connect to SQL Server <servername>\VEEAMSQL2016. A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 – Error Locating Server/Instance Specified)
To make it even more strange the default Microsoft SQL Server instance name (and database name) is already entered when selecting the SQL Server engine, suggesting that it will install that.Veeam Backup Enterprise Manager with Microsoft SQL Server configuration database does not support the management of backup servers using PostgreSQL.
Is there really no Microsoft SQL Server Express install included anymore? This will generate a number of issues for 'enterprise size' customers as it is impossible to do a "big bang" migration to PostgreSQL. Note that a lot of larger customers have waited until v12 is proven stable (now with v12.1) before they migrate to v12.