MS SQL server listed as installed in Home > Tools & Settings > Database Servers :
Microsoft SQL Server .\MSSQLSERVER2012 (default for MS SQL)
However, it is missing in SQL Service section in Home > Tools & Settings > Plesk Components.
Microsoft SQL Server support feature is disabled in the currently installed Plesk License.
Check it at Home > Tools & Settings > License Management. There should be line as the following:
Microsoft SQL Server support On
If it is in
Off state edit key features in Key Administrator or contact your sales representative to have Microsoft SQL Server support feature enabled.
If it is enabled in license key and still missing in Plesk Components re-install it using Plesk installer.