CADmep runs on top of AutoCAD. 

If you want to rum CADmep 2019, you need AutoCAD 2019 or one of the AutoCAD based products (AutoCAD Arch, AutoCAD MEP, AutoCAD Elect, etc.)

CADmep and AutoCAD versions "Should" match. Some versions will allow you to run different versions of CADmep and AutoCAD but 2019 is not one of them.