Hi,
You can only upgrade to System Center 2019 from System Center 2016 or 1801 or 1807.
Plan the upgrade to System Center 2019 - Service Manager
To upgrade to System Center 2016, an in-place upgrade from Service Manager 2012 R2 to Service Manager 2016 is supported. An in-place upgrade is an upgrade of all Service Manager parts on the same hardware.
Upgrade Path: 2012 R2 -> 2016 -> 2019